BACKGROUND: Large-scale studies performed outside the United States have demonstrated that most cases of onychomycosis and tinea pedis are caused by dermatophytes, primarily Trichophyton rubrum. However, other studies have suggested that yeasts and nondermatophytic molds may play a role, particularly in onychomycosis. OBJECTIVE: This study was undertaken to determine the epidemiology of superficial fungal infections in a U.S. population. METHODS: Fungal cultures were performed on patients with clinically suspected tinea cruris, tinea corporis, tinea capitis, tinea pedis, and onychomycosis. RESULTS: Dermatophytes were the most commonly isolated fungi in each type of superficial fungal disease studied. T. rubrum was the most commonly isolated dermatophyte species, although Trichophyton tonsurans was more common in tinea capitis and equally common in tinea corporis/tinea cruris. In tinea pedis and onychomycosis, dermatophytes appeared in approximately 95% and 82% of isolates, respectively. Candida albicans and nondermatophyte molds played only a minor role in onychomycosis; C. albicans was isolated in 7% of nail cultures and nondermatophytic molds were isolated in 11%. CONCLUSION: These results are in general agreement with other major epidemiologic studies performed outside the United States. Dermatophyte fungi cause most superficial fungal infections.