Onychomycosis is caused primarily by dermatophytes, Candida species, and nondermatophytic molds. Dermatophytes, particularly Trichophyton rubrum , are by far the most common pathogens. There is some question as to whether Candida actually breaks down nail material or only invades the proximal nail bed. Similarly, the clinical significance of molds is unknown, because they may be colonizing organisms rather than destructive pathogens. It is, therefore, important to identify the pathogen in the array of organisms that may be isolated in culture. The increasing use of fluconazole in prophylaxis and treatment of systemic yeast infections and infections in patients with AIDS has been associated with the emergence of resistant Candida albicans , as well as previously minority species of Candida , such as C. glabrata and C. krusei . This may be of relevance to the treatment of onychomycosis with azole antifungals. (J Am Acad Dermatol 1998;38:S32-6.)