Periungual fibromas are small, soft, and compressible papules typically found adjacent to the proximal or lateral nail folds; in the context of tuberous sclerosis (TSC), they are also called Koenen tumors. They are rarely seen in the general population (more commonly isolated, likely due to trauma) and are seen in 15%-80% of patients with TSC (second-hit mutation allelic deletion of TSC2).

Onset of periungual fibromas is typically in the second decade and as late as the fifth decade of life. Periungual fibromas are rarely reported in children younger than 5 years. No differences between sexes or among different ethnicities or races have been reported. Periungual fibromas are occasionally painful, and some are precipitated by trauma. They are more common on the toes than fingers.

Nontraumatic periungual fibromas are a major diagnostic criterion for the diagnosis of TSC. However, patients with a preceding history of trauma should also be examined for manifestations of TSC. In TSC, periungual fibromas are usually the last skin finding to present and subungual fibromas are a further, less frequent finding. "Red comets" (erythronychia) and longitudinal grooves may be found in the nails of patients with TSC, and these findings should increase clinical suspicion of TSC, even in the absence of periungual fibromas.

There is one case report of a periungual fibroma in a patient with Birt-Hogg-Dubé syndrome.