The study aimed to evaluate the effectiveness of metronidazole 0.75% gel in patients with mild and moderate seborrhoeic dermatitis. Sixty-seven patients with seborrhoeic dermatitis were enrolled. Cases were randomly treated with metronidazole 0.75% gel or placebo for four weeks and were additionally followed up for another four weeks. Patients were evaluated by scoring before the treatment, once a week during the treatment and twice after the cessation of the treatment within a 15-day interval. Furthermore, patient satisfaction and doctor global evaluation were done at the end of the treatment and of the study as well. In the metronidazole group 33 patients (median age: 26, total severity score: 15.0 +/- 11.0 (median +/- interquartile range) and in the placebo group 34 patients (median age: 26, total severity score: 13.0 +/- 7.5) were enrolled in the study. Three patients from the metronidazole group and four patients from the placebo group did not attend to follow-up visits. Erythema, scales, papule, pruritus and the total severity scores in both group decreased significantly during the treatment when compared with the basal levels (p < 0.05). There was no difference between the two groups in terms of efficacy (p > 0.05). Total severity scores were found as 7.33 +/- 1.08 and 6.43 +/- 0.93 in the metronidazole and placebo groups at the end of the treatment, respectively. After the cessation of the treatment, all scores had increased rapidly. Total severity scores were 10.40 +/- 1.54 and 11.20 +/- 1.53 in the metronidazole and placebo groups one month after the cessation of the treatment, respectively. Both metronidazole 0.75% gel and the placebo were well tolerated by the patients. In conclusion, in the treatment of seborrhoeic dermatitis, administration of metronidazole 0.75% gel is well tolerated but it is only as effective as placebo and the disease severity quickly returns to the basal levels after the cessation of treatment.