Seventy patients (36 males and 34 females) with seborrhoeic dermatitis of the scalp were treated in a double-blind controlled study, for a maximum of 6 weeks, with 2% miconazole base and 1% hydrocortisone (Daktacort), 2% miconazole base, or 1% hydrocortisone. Patients who were cured were treated with the same formulation prophylactically twice monthly for 3 months or until recurrence. Nineteen of 21 patients were cured in the Daktacort group, 15 of 22 in the miconazole group and 17 of 24 in the hydrocortisone group. The number of cultured Pityrosporum orbiculare was significantly lower in all groups after treatment, but in the hydrocortisone group was still significantly higher than in the two other groups. After 3 months of prophylactic treatment, both Daktacort (16 of 19 patients clear) and miconazole (10 of 15 patients clear) were significantly better than hydrocortisone (3 of 17 patients clear) (P less than 0.01). The numbers of P. orbiculare remained low in the Daktacort and miconazole groups and also significantly lower than in the hydrocortisone-treated group (P less than 0.01). In patients with recurrence, the numbers returned to pre-treatment levels. This study demonstrates the aetiological significance of the Pityrosporum yeasts in seborrhoeic dermatitis. Both Daktacort and miconazole were effective in treatment and as prophylactic agents.