What causes a pilar cyst?

These cysts are also called trichilemmal cysts. They commonly feel like smooth, firm, rounded lumps beneath the skin and may move slightly when touched. The cause is linked to the follicle wall and can sometimes run in families, so more than one scalp cyst may occur. A cyst can remain quiet, become tender after pressure, or become inflamed if its contents irritate nearby tissue. A scalp lump has several possible explanations, so appearance alone cannot confirm the diagnosis.

Avoid squeezing, puncturing, or repeatedly rubbing the lump; these actions can injure the skin and introduce infection. Keep the area clean and note its size, tenderness, drainage, and any change in the overlying skin. A clinician can examine it and decide whether observation is suitable. If it is painful, repeatedly inflamed, catches on clothing or grooming tools, or affects daily life, removal of the whole cyst and its sac may be discussed. Learn more about [Cyst Removal](/treatments/surgical-dermatology/cyst-removal/).

Arrange a prompt assessment if the lump becomes rapidly larger, hot, very painful, markedly red, or starts draining pus, especially with fever. Seek urgent help for severe swelling around the eye, confusion, or feeling very unwell. A new, changing, bleeding, or persistently ulcerated scalp lump also deserves examination rather than self-treatment. You can review related symptoms under [Skin Lumps](/symptoms/skin-lumps/).

Could this be a pilar cyst or another type of scalp lump? What signs would make observation unsafe? If removal is considered, would the entire cyst sac be taken out, and how should the wound be cared for afterward?