Clothes that smell like pee usually pick up that ammonia-like odor from one of a few sources: your own sweat, a lingering laundry problem, a pet, or less commonly, a medical condition. The good news is that most causes are fixable once you identify what’s going on.
Sweat Contains Ammonia Naturally
Your body produces two types of sweat. The kind that covers most of your body (eccrine sweat) actually contains ammonia as a normal component. The other type (apocrine sweat, concentrated in your armpits and groin) starts out odorless but gets broken down by bacteria living on your skin into ammonia and fatty acids. That combination is what gives worn clothes their sharp, urine-like smell.
The ammonia in your sweat increases when your body is burning more protein for fuel. This happens during intense exercise, on high-protein or low-carb diets, or when you’re dehydrated. If you’ve recently changed your diet or exercise routine and noticed the smell getting worse, that connection is worth paying attention to. Drinking more water dilutes the ammonia your body produces, and eating enough carbohydrates gives your muscles an alternative fuel source so they break down less protein.
Synthetic Fabrics Trap Odor Differently
If the smell is worse in your gym clothes, activewear, or polyester shirts, the fabric itself is part of the problem. Bacteria cling to synthetic fibers more stubbornly than to cotton or wool, and they form protective layers called biofilms that help them survive washing. These biofilms let bacteria stick to the fabric even after a full wash cycle. Research published in BMC Biology found that the number of living bacteria on clothing actually increases after washing, partly because biofilm-forming species from the washing machine colonize the fabric during the cycle.
Once established, these bacteria reactivate the moment conditions are right. When you put on a “clean” synthetic shirt and start sweating, the bacteria wake up, feed on your skin oils and sweat residue trapped in the fabric, and immediately start producing the same ammonia and fatty acid compounds that made the shirt smell in the first place. This is the “permastink” phenomenon: odor that builds up over repeated wear-wash-dry cycles and never fully goes away.
How Your Laundry Routine Makes It Worse
Leaving wet or damp clothes sitting before they dry is one of the fastest ways to develop a urine-like smell. Bacteria and fungi multiply rapidly in humid conditions, and during slow air drying, their numbers can spike significantly. If you regularly leave laundry sitting in the washer after the cycle ends, or if your clothes dry slowly in a humid room, those microorganisms are actively producing odor compounds the entire time.
A few changes can break the cycle. Wash sweaty clothes promptly rather than letting them sit in a hamper for days. Use the hottest water the fabric allows, since heat kills more bacteria. Add white vinegar to the rinse cycle to help neutralize ammonia. Dry clothes quickly, ideally in a dryer or in direct sunlight, which research shows reduces microbial growth. For synthetic fabrics with embedded odor, soaking in a vinegar or baking soda solution before washing can help break down biofilms that regular detergent misses.
Pets Are a Common Hidden Cause
If you have cats or dogs, pet urine may be transferring to your clothes in ways you haven’t noticed. A pet that has urinated on a laundry pile, a bed, or a couch cushion can leave behind urine residue that migrates to your clothing through contact. Pet hair and dander carry bacteria that can also transfer to human clothes during a shared wash load. Even small amounts of dried urine are enough to produce a noticeable smell, especially once the fabric gets warm from your body heat.
Check spots where your pets rest or where laundry touches the floor. A black light can reveal dried urine stains that aren’t visible otherwise. Wash pet-contaminated items separately, and use an enzyme-based cleaner or detergent, which breaks down the uric acid crystals that give pet urine its staying power. Standard detergent often leaves those crystals intact.
Medical Conditions That Change Your Smell
Sometimes the source of the smell is your body, not your laundry routine. A urinary tract infection is one of the more common medical causes. Certain bacteria involved in UTIs, particularly Proteus mirabilis, produce an enzyme that breaks urea down into ammonia. This raises the ammonia concentration in your urine significantly, and even small amounts of leakage you might not notice can leave a strong smell on underwear or pants.
Urinary incontinence is another straightforward explanation that people sometimes overlook. Stress incontinence (small leaks when you cough, sneeze, laugh, or exercise) affects roughly one in three women at some point and is more common than many people realize. Even tiny amounts of urine on fabric will smell stronger as the day goes on, because bacteria on the fabric continue breaking down the urea into ammonia.
Uncontrolled diabetes can also change the way your urine and sweat smell. When blood sugar stays elevated, sugar and ketones accumulate in urine and create a strong, sometimes sweet odor. Advanced diabetes produces a distinct smell that can linger on clothing. Kidney problems can cause a similar effect, since your body may start excreting more waste products through sweat when the kidneys aren’t filtering efficiently.
A rarer condition called trimethylaminuria causes the body to produce a compound that smells fishy or foul. At lower concentrations, this can be perceived as a general unpleasant or “garbage-like” smell rather than specifically fishy. The compound builds up in sweat, urine, and breath, and it’s linked to eating foods high in choline, such as eggs, organ meats, saltwater fish, and certain legumes. Most people metabolize choline without issue, but those with a genetic variant in the enzyme that processes it can accumulate the odor-causing compound.
How to Narrow Down the Cause
Start with the simplest explanations. Smell your clothes right out of the dryer. If they already smell, the problem is in your laundry process: your washer may need cleaning, your clothes aren’t drying fast enough, or biofilms have built up in synthetic fabrics. If clothes smell fine out of the dryer but develop the odor during the day, the source is your body, whether that’s normal sweat, diet-related changes, or a medical issue.
Check whether the smell is limited to certain garments. If it’s only in underwear or pants, consider urinary leakage or a UTI. If it’s strongest in shirts, especially in the underarm area, sweat and bacteria are the more likely culprits. If every fabric in your home has a faint smell, a pet marking territory or a hidden urine stain somewhere in the house may be the root cause.
For persistent odor that doesn’t improve with better laundry habits and isn’t explained by pets or diet, it’s worth getting a basic checkup that includes a urinalysis. A urine test can screen for infection, diabetes, and kidney function in one simple step, and it rules out the medical causes that are easiest to treat.