Onychomycosis, by definition, is a mycotic infection of the keratinized tissue of the nail plate. Although it is commonly considered to be caused by one of the dermatotropic fungi, a variety of other organisms have been implicated as etiologic agents in the disease, including some bacteria and yeasts. When it is caused by a fungus, any or all of three types of organisms can be involved: dermatophytes, yeasts, and nondermatophyte organisms. The purpose of this study was to identify the microorganisms found in fungal cultures of clinically suspected onychomycosis in the patient population of the Foot Clinics of New York in New York City, the largest foot clinic in the world. Of the 1,800 medical charts reviewed, 214 had culture results, of which 120 were positive. Trichophyton rubrum was the most prevalent pathogen, found in 67% of positive cultures. The most remarkable risk factor was age, with 80% of affected individuals older than 35 years. False-negatives may account for the high percentage (44%) of negative cultures in this study.