Trichomycosis axillaris is an asymptomatic superficial bacterial infection caused by Corynebacterium spp. It is relatively common and is the result of bacterial overgrowth, with concretions forming on hair shafts in moist regions of the body. These concretions represent bacterial colonies and the insoluble cement substance elaborated by the bacteria that adheres to the hair shaft.

Trichomycosis axillaris affects chiefly the hair of the axilla but may, to a lesser extent, involve the pubic hair as well (trichomycosis pubis). It may affect any age group from puberty through adulthood. The condition appears to affect men more often, likely because many women shave the axillary hair.

Recent evidence suggests that patients with one cutaneous disease process caused by Corynebacterium, such as trichomycosis axillaris, may often have other related and coincidental disease processes caused by the same bacterial species, such as pitted keratolysis (a corynebacterial infection of the feet) or erythrasma (a corynebacterial infection of the intertriginous skin).