Background: Fluconazole is a bis-triazole antifungal agent approved for the treatment of oropharyngeal, esophageal, and vaginal candidiasis, serious systemic candidal infections, and cryptococcal meningitis. Objective: The purpose of this study was to evaluate three different durations of once-weekly fluconazole for the treatment of onychomycosis of the toenail caused by dermatophytes. Methods: In a multicenter, randomized, double-blind, parallel, placebo-controlled trial, 384 patients with distal subungual onychomycosis of the toenail received fluconazole, 450 mg once weekly, or placebo for 4, 6, or 9 months. For inclusion, patients were required to have mycologically confirmed distal subungual onychomycosis of the toenail with a large toenail at least 25% clinically affected but having at least 2 mm of healthy nail between the nail fold and the proximal onychomycotic border. Efficacy was assessed by clinical and mycologic (microscopic and microbiologic) measures at screening, at every treatment visit starting at month 3, and at months 2, 4, and 6 after therapy. Observed or volunteered adverse events were recorded and classified at all visits. Results: At the end of treatment, very significantly superior clinical and mycologic results were achieved in all fluconazole groups compared with placebo (p = 0.0001). This superiority was largely maintained over 6 months of follow-up. The clinical and mycologic responses of the 9-month treatment duration were significantly superior to the 4- and 6-month durations. Similar percentages of patients in the fluconazole and placebo groups reported adverse experiences for all three durations of the study. Conclusion: Results of this study support the efficacy and safety of fluconazole in the treatment of distal subungual onychomycosis of the toenail.(J Am Acad Dermatol 1998;38:S95-102.)