Sixty-two patients with seborrhoeic dermatitis were treated topically with a 2% ketoconazole foaming gel or with a 0.05% betamethasone dipropionate lotion in a single-blind study for 4 months. Changes in the number of Pityrosporum ovale were scored by a mycologist. The investigator rated the severity of erythema, scaling and itching of the patients' scalp, eyelashes, nasolabial folds and thorax. In addition, both the investigator and the patients evaluated the treatments globally. At the end of treatment, the response rate for ketoconazole 2% foaming gel was significantly higher than that for betamethasone dipropionate 0.05% lotion according to the global evaluation by the physician (89 vs. 62%, p less than 0.05) and the patient (89 vs. 65%, p less than 0.05). Ketoconazole was also superior to betamethasone with reference to the evolution of the symptoms, irrespective of their localization. This efficacy manifested itself by a significant reduction of the number of P. ovale on the scalp in the ketoconazole group (p less than 0.001) compared to the betamethasone group, in which the count was hardly changed during therapy. The treatment was also better tolerated in the ketoconazole group (5 vs. 16 patients with side-effects, p less than 0.001). It is concluded that ketoconazole 2% foaming gel offers an excellent alternative to local corticosteroids in the treatment of seborrhoeic dermatitis.