If you’ve noticed a small red lump near the top of your buttock cleft — especially if there’s a strand poking out of it — you might be wondering, what on earth is that?

That dark bit you’re seeing is actually a tuft of hair, sitting in the opening of a pilonidal sinus (that’s the small red bump above the buttock crack in the photo). The hair didn’t grow there — it worked its way in through tiny midline holes lower down in the cleft (you can see those below in the photo).

Here’s what’s happening beneath the surface:

How the Hair Gets In

• It’s not an “ingrown hair.” Hairs have tiny scales that point toward the tip, which means they can only insert root-end first.
• They enter through small midline pits. These pits are like open doorways in the skin at the base of the cleft. Loose or cut hairs can slide right in, especially when they’re short and stiff.
• A perfect storm of suction and motion. Every time you stand up, sit down, or bend over, the skin over your tailbone stretches and moves. This creates a mini “vacuum effect,” helping draw hairs into those pits. Repeated movement of the buttocks then pushes the hairs deeper over time.

Why It Becomes a Problem

As more hairs get trapped under the skin, they bunch together — a little “nest of hairs.” (In fact, pilonidal literally means “nest of hairs.”)

Because the body doesn’t like foreign objects, it launches an inflammatory response. The area becomes irritated or infected, and you may end up with a cyst or abscess — the swollen, sometimes painful lump you’re seeing.

In a Nutshell

A pilonidal sinus is essentially your body’s reaction to trapped hairs that have migrated under the skin of the buttock cleft. It’s a mechanical and anatomical issue — not about poor hygiene or ingrown hairs.

That is Pilonidal Sinus 101!