In a series of 60 patients (42 males, 18 females) with widespread lesions of pityriasis versicolor, 200 mg of oral ketoconazole was administered daily for a period of 24 days. Routine laboratory examinations, including microscopic and Wood's light examinations, were performed prior to initiation of treatment and twice during the treatment period as well as 3 and 6 months, and 1 and 2 years after completion of treatment. In 95% of the patients the clinical cure was maintained after 3 months but after 6 months it was still present in only 80% and after one year in only 57%. At the end of two years 60% showed relapse of the disorder. In 75% hypopigmented macules persisted after 6 months but without microscopic evidence of the fungus. No side effects were reported. It is evident that oral ketoconazole is effective in maintaining a clinical cure mainly during the first 3 months after treatment. The question is to whether side effects might appear with a more prolonged course of the drug should be given serious consideration before continuation of treatment is contemplated.