Proximal subungual onychomycosis is a type of fungal nail infection in which invasion begins from the undersurface of the proximal nail fold and then progresses distally. The nail changes are either diffuse patches or transverse striate patterns. The striate pattern may be associated with superficial onychomycosis. When proximal subungual onychomycosis progresses rapidly, it may be associated with the human immunodeficiency virus (HIV) / AIDS. It is the least common form of onychomycosis in immunocompetent individuals.

The dermatophyte fungus Trichophyton rubrum is the most common causative agent. Other organisms include Candida, Fusarium, and Aspergillus.

Paronychia may lead to proximal nail plate invasion. Candida and Fusarium are often responsible.

The entire nail may eventually become involved. Toenails are more commonly involved than fingernails. Spread to involve multiple toenails often occurs without treatment. Concomitant nail fold infection (paronychia) may lead to pain and discomfort.

Males are affected more often than females, but there is no ethnic predilection. Adults are affected more often than children.