Proximal subungual onychomycosis (PSO) is the rarest form of onychomycosis and most commonly due to Trichophyton rubrum, The fungus initially penetrates the horny layer of the proximal nail fold and subsequently invades the under-surface of the nail plate. PSO has been mainly reported in patients with AIDS and AIDS related complex and may serve as an early clue for the presence of imrnunodeficiency. We report a case of PSO in a 45-year-old woman with reumatoid arthritis who received oral steroid. (Kor J Dermatol 1996;34(2): 341-344)