Hidradenitis suppurativa (HS) starts as deep, painful lumps under the skin that can look like boils but don’t behave like them. These nodules typically measure 0.5 to 2 cm across and develop in areas where skin rubs together: the armpits, groin, buttocks, and under the breasts. Over time, the condition can progress to include draining tunnels beneath the skin, open wounds, and thick scars. About 2.5% of the global population has HS, yet it takes an average of several years to get a correct diagnosis, partly because the early stages mimic other conditions.
Early Lesions: Deep Nodules and Abscesses
The first sign of HS is usually a firm, deep-seated nodule buried under the skin. These aren’t surface-level pimples. They sit deep in the tissue, feel tender or painful to the touch, and can persist for days to months without resolving. You might see just one at first, or a few scattered across one area. The skin over the nodule may appear swollen and inflamed, but the lump itself can be hard to see in its earliest stages because it forms well below the surface.
These nodules can eventually become abscesses, filling with fluid that may rupture and drain. The fluid is often thick, sometimes foul-smelling, and can leave stains on clothing. One key detail that separates HS abscesses from regular skin infections: when doctors culture the fluid from unruptured HS lesions, the results typically come back sterile. This isn’t a bacterial infection in the traditional sense. It’s a chronic inflammatory process driven by the immune system, which is why antibiotics alone rarely resolve it.
How It Differs From Boils
Many people with HS are initially told they have recurring boils (furuncles) or ingrown hairs. The visual overlap is real, but several features set HS apart. Regular boils tend to be superficial, come and go without a predictable pattern, and heal cleanly. HS nodules are deeper, recur in the same body areas repeatedly, and are prone to rupturing and forming interconnected tunnels beneath the skin that can ulcerate or drain. If you’ve had painful lumps in your armpits or groin that keep coming back in the same spots, especially if they’ve started leaving scars, that pattern points toward HS rather than simple boils.
Tunnels and Draining Wounds
As HS progresses, some nodules connect to each other beneath the skin through channels called sinus tracts or tunnels. These are one of the most distinctive visual features of the condition. On the surface, you may see multiple openings that ooze fluid, sometimes at a distance from each other, connected by firm, cord-like tissue you can feel under the skin. The drainage can be persistent, requiring frequent bandage changes and causing significant discomfort in daily life.
These tunnels form because the inflammation repeatedly destroys the hair follicle structures deep in the skin, and the body’s attempt to repair the damage creates abnormal pathways rather than normal healing. Once established, tunnels rarely resolve on their own and often require surgical intervention.
Scarring and Chronic Skin Changes
HS lesions don’t heal like a normal cut or scrape. They leave behind distinctive scars that worsen over time with repeated flares. The scarring takes several forms:
- Atrophic scars: shallow, sunken areas where tissue has been lost, sometimes with a pitted or sieve-like (cribriform) texture
- Hypertrophic scars: raised, firm plaques of scar tissue, sometimes forming thick rope-like bands across the affected area
- Contractures: tight bands of scar tissue that can restrict movement, particularly in the armpits or groin
Another late-stage feature is the “tombstone” comedone, a distinctive type of open blackhead with two or more heads. These multiheaded comedones form after repeated inflammatory destruction of the hair follicle unit and are considered an end-stage marker of the disease. They look different from ordinary blackheads because they’re larger, often double-ended, and clustered in areas of previous inflammation.
What It Looks Like on Different Skin Tones
Most clinical images of HS show it on lighter skin, where active inflammation appears pink or red. On darker skin tones, the same inflammation often looks violet, grayish, or brown instead. This difference can make it harder to spot active flares at a glance, both for patients and for clinicians unfamiliar with how the condition presents across skin tones.
A useful clue on darker skin: look for areas of post-inflammatory hyperpigmentation (darkened patches) in typical HS locations like the armpits, groin folds, and inner thighs. These dark patches indicate where previous inflammation has occurred, even if the original redness wasn’t obvious. Over time, the combination of active lesions, darkened patches, and scarring creates a mottled appearance in affected areas.
The Three Stages of Severity
Doctors classify HS appearance using the Hurley staging system, which describes three levels based on what the skin looks like at a given time.
Stage I involves one or more abscesses without any tunnels or scarring. This is the mildest form and the one most often confused with ordinary boils. The skin between flares may look completely normal, making it easy to dismiss.
Stage II features recurrent abscesses that are widely separated, with visible sinus tracts and scarring beginning to develop between them. At this point, the affected area starts to show permanent changes even between flares, with firm scar tissue and possibly a few draining openings.
Stage III is the most severe presentation. The involvement is diffuse or near-diffuse, meaning multiple interconnected tunnels and abscesses cover large areas with little to no uninvolved skin remaining. The affected region may appear as a continuous landscape of swollen tissue, open wounds, draining tracts, and thick scarring. Movement in the area can be restricted, and the condition at this stage significantly impacts daily functioning.
Where It Appears on the Body
HS develops almost exclusively in intertriginous areas, the places where skin folds meet and rub. The most common locations are the armpits, the groin and inner thighs, the buttocks and perianal area, and underneath the breasts. Some people develop lesions in less typical spots like the waistband area, behind the ears, or on the nape of the neck, but these are secondary locations.
The distribution often provides a diagnostic clue. If painful, recurring lumps keep showing up in your armpits or groin rather than on your face, back, or other typical acne locations, that pattern is characteristic of HS. Many people have involvement in more than one body region at the same time, and the condition tends to be roughly symmetrical, affecting both armpits or both sides of the groin.