Forty patients (22 males and 18 nonpregnant females) with tegumentary mycoses were treated with ketoconazole (R41,400). The group included 39 patients with dermatophytoses and one with tinea versicolor. Ketoconazole was administered in one dose per day taken with water 2 hr before or after breakfast for one month; patients weighing 30 kg received 200 mg per day. Twenty-one patients had complete clinical and mycologic cure, two responded clinically but the last culture was positive, eight had partial improvement, and three had no improvement at all. In six cases the treatment was stopped (in one because of gastric intolerance). The main adverse effect of ketoconazole was nausea; only one patient had vomiting. The results indicate that ketoconazole is a safe and effective drug for treatment of dermatomycosis.