Hidradenitis suppurativa is a skin condition that causes lumps in areas where skin rubs against skin, such as in the armpit. The lumps look like pimples, acne, or boils. The lumps are usually painful and can become infected.

The lumps may come and go for years. If the condition isn’t treated and gets worse, hollow tunnels can form under the skin. Over time, the infection and tunnels will heal, but a thick scar may form. These scars can keep skin from stretching naturally.