Ketoconazole was administered orally in daily doses of 200 mg to 70 patients with onychomycosis. Complete recovery was attained by 48 patients, improvement of >50% by 10 patients, and for three patients therapy failed. For the remaining nine patients the results could not be evaluated. The average duration of treatment was 7.5 months for disease due to Trichophyton species and 6.5 months for disease due to Candida species. There were no adverse effects or signs of toxicity attributable to the administration of ketoconazole. It is concluded that ketoconazole is a positive development in the effort to control the difficult problems presented by onychomycosis.