The results of treatment with ketoconazole of 1,104 patients in Europe and the Americas with superficial or deep mycoses are presented. Clinical and mycologic responses to treatment were measured. One dose of ketoconazole daily was highly effective in the treatment of superficial mycoses, including chronic mucocutaneous candidosis, caused by dermatophytes and yeasts. Of the deep mycoses, the highest response rates were in paracoccidioidomycosis and histoplasmosis. Ketoconazole was moderately effective in the treatment of coccidioidomycosis, chromomycosis, and aspergillosis but had little or no effect in aspergilloma or mycetoma. Preliminary data are available for systemic candidosis, cryptococcosis, sporotrichosis, blastomycosis, lobomycosis, and infection due to Alternaria species. The lengths of treatment necessary for cure are as follows: vaginal candidosis, three to five days; oral thrush, one to two weeks; pityriasis versicolor and dermatomycoses caused by yeasts, one to six weeks (median, three); dermatomycoses caused by dermatophytes, two to eight weeks (median, four); mycoses of hair and scalp, one to two months; paracoccidioidomycosis and histoplasmosis, one to six months (median, two); and onychomycosis and chronic mucocutaneous candidosis, one to 12 months (median, five). The response to ketoconazole seemed to be independent of the response to other antifungal drugs and associated immune suppression. Diabetes mellitus seemed to reduce responsiveness of deep mycoses but not of superficial mycoses to ketoconazole. Preliminary data from four studies of patients with cancer indicate that ketoconazole prevents mycotic infections in patients with impaired host defenses and reduces the colonization of the gastrointestinal tract. Ketoconazole was well tolerated, and no abnormalities in hematologic or biochemical indexes were noted.