Terbinafine is an allylamine antifungal compound shown to be effective in the oral treatment of onychomycosis. Because of the fungicidal activity of the drug, a shorter duration of treatment, compared with the currently used oral treatment modalities, can be expected in onychomycosis of the toenail. In the present randomized study, the efficacy of oral terbinafine treatment (250 mg/day) was assessed for periods of 6, 12, and 24 weeks. All patients were followed for up to 48 weeks after starting treatment. Of the 120 patients with toenail onychomycosis who entered the study, 98 were evaluable for efficacy. The involvement of the toenails was assessed both clinically and mycologically throughout the study. Evaluation at 24 weeks showed that complete cure of toenail onychomycosis was achieved in 67% of patients treated for 6 weeks, 82% treated for 12 weeks, and 85% treated for 24 weeks. At the end of a further 24 weeks of follow-up, cure rates were 40%, 71% and 79%, respectively. The adverse effects of terbinafine were mostly mild-to-moderate gastrointestinal symptoms. Three patients discontinued treatment because of side-effects. In conclusion, oral treatment with terbinafine is effective and generally well tolerated in patients with onychomycosis. Our results demonstrate that, for toenail onychomycosis, a treatment period of 12 weeks is sufficient.