A double-blind, placebo-controlled study was undertaken to determine the efficacy of ketoconazole in the treatment of candidiasis. The drug was administered orally in the dosage of 200 mg/m2 of body surface per day for two weeks to cancer patients with oral candidiasis. Randomization in a 2:1 ratio provided 36 patients treated with ketoconazole and 20 managed with a placebo. Regression of visible lesions was achieved in 26 (72%) of 36 ketoconazole-treated and four (20%) of 20 untreated patients; eradication of culturable organisms occurred in 12 (36%) of 33 ketoconazole-treated and one (7%) of 14 untreated patients; and resolution of lesions plus eradication of Candida albicans occurred in nine (25%) of 36 ketoconazole-treated and one (5%) of 20 untreated patients. Although the therapeutic efficacy of ketoconazole was demonstrated for oropharyngeal candidiasis, the magnitude of its efficacy was less than that desired.