A randomized, double-blind, placebo-controlled study was made of 2% ketoconazole cream and shampoo in 20 patients with seborrhoeic dermatitis of the face. Sixteen also had seborrhoeic dermatitis of the scalp and five had seborrhoeic dermatitis of the chest or back. Responses were measured by clinicians and patients independently using a grading system and linear analogue scales, respectively. Face and scalp lesions, assessed by both patient and clinician, showed a significant improvement or complete clearance in the group treated with ketoconazole. The patients who had seborrhoeic dermatitis of the chest or back and were treated with ketoconazole also improved. There was no improvement with placebo. This study provides further evidence for the aetiological role of pityrosporon yeasts in seborrhoeic dermatitis and of the efficacy of topical ketoconazole in its treatment.