BACKGROUND Griseofulvin has been used in the treatment of toenail onychomycosis with limited success. Evidence suggests that terbinafine may be more effective. OBJECTIVE In a double-blind, parallel-group study we compared 250 mg/day terbinafine for 16 weeks with 500 mg/day griseofulvin for 52 weeks (or for shorter periods in cured patients) in patients with toenail onychomycosis. METHODS Eighty-nine patients with culture-proved tinea unguium were included, and 43 in the terbinafine group and 41 in the griseofulvin group were assessable for efficacy. Patients who had not improved after 16 weeks were entered into an open study and were given 250 mg/day terbinafine for 16 weeks with the study code still blinded and were then followed up for 20 weeks. RESULTS Terbinafine was significantly more effective than griseofulvin, with 42% being completely cured and 84% mycologically cured compared with only 2% with total cure and 45% with mycologic cure in the griseofulvin-treated group. The number of side effects was significantly lower in the terbinafine group (11%) compared with the griseofulvin group (29%). CONCLUSIONS Terbinafine is significantly more effective than griseofulvin in the treatment of toenail onychomycosis.