Background: Onychomycosis is an increasing problem with limited therapeutic options. Objective: We evaluated the safety and efficacy of oral terbinafine, a new fungicidal antimycotic, in patients with toenail onychomycosis. Methods: A North American multicenter, double-blind, placebo- controlled study evaluated the mycologic and clinical efficacy of oral terbinafine 250 mg/day for 12 or 24 weeks in 358 patients with toenail onychomycosis. Results: A total of 74% of patients treated with 12 or 24 weeks of terbinafine achieved a successful clinical outcome. Approximately 11% of terbinafine responders showed evidence of relapse 18 to 21 months after cessation of treatment. Terbinafine was well tolerated; most adverse events were transient and mild to moderate in severity. Conclusion: The results of this study confirm that oral terbinafine is a safe and effective therapy for the treatment of onychomycosis.