Nineteen patients with chronic dermatophyte infections were treated with 200 to 400 mg per day of ketoconazole orally. Sixteen had had their disease for 2 years or more. Sixty-three per cent achieved cure, and an additional 26% showed clinical improvement. Clearing of lesions occurred in 10 weeks (mean). The dose was increased to 600 mg per day in four patients. Of these, two achieved cure, for an overall cure rate of 74%. Seventy-five percent of patients who previously failed to respond to griseofulvin were cured or improved by ketoconazole. Four of eight patients with onychomycosis were cured, and the rest improved on therapy. One patient developed a generalized drug eruption after the first dose of ketoconazole, and possible minor side effects were noted in five. Three of eight patients followed off therapy after apparent cure subsequently showed relapse. The drug appears to have great potential value for the therapy of dermatomycoses.