We have treated 48 cases of onychomycosis (of which 37 were caused by dermatophytes, 10 by yeasts and one by Scopulariopsis brevicaulis) with 200 mg ketoconazole daily. We obtained recovery in 65 p. 100 of the cases of onyxis caused by dermatophytes and in 80 p. 100 of the cases of onychomycosis due to Candida. The one patient presenting an infection with Scopulariopsis brevicaulis recovered in 13 months. The average duration necessary to obtain complete recovery was 6 1/2 months for onychomycosis of the hands due to dermatophytes and 12 1/2 months for those of the feet. Perionyxis due to Candida needed 2 months of treatment with this drug, however 6 months of treatment were necessary to obtain recovery for onycholysis due to Candida. Biological tests remained normal and the side-effects were minimal and essentially gastrointestinal in our study. Ketoconazole is an effective treatment for onychomycosis: it is active against the different mycotic agents infecting nails and well tolerated by the patient. Several minor effects such as itching, nausea, headache and more serious reactions such as erythrodermia and hepatitis have been reported. Regular control and biological tests are therefore necessary. Patients with other diseases should avoid the use of ketoconazole for treatment of onychomycosis.