Background: Onychomycosis is a prevalent infection of the nail caused primarily by dermatophytes. Fluconazole is active in vitro against the most common pathogens of onychomycosis, penetrates into the nail bed, and is clinically effective in the treatment of a wide variety of superficial fungal infections. Objective: The purpose of this study was to compare the efficacy and safety of three different doses of fluconazole (150, 300, and 450 mg) given orally once weekly to that of placebo in the treatment of distal subungual onychomycosis of the toenail caused by dermatophytes. Methods: In this multicenter, double-blind study, 362 patients with mycologically confirmed onychomycosis were randomized to treatment with fluconazole, 150, 300, or 450 mg once weekly, or placebo once weekly for a maximum of 12 months. To enter the study, patients were required to have at least 25% involvement of the target nail with at least 2 mm of healthy nail from the nail fold to the proximal onychomycotic border. Patients who were clinically cured or improved at the end of treatment were further evaluated over a 6 month follow-up period. At both the end of therapy and the end of follow-up, clinical success of the target nail was defined as reduction of the affected area to less than 25% or cure. Results: At the end of therapy, 86% to 89% of patients in the fluconazole treatment groups were judged clinical successes as defined above compared with 8% of placebo-treated patients. Clinical cure (completely healthy nail) was achieved in 28% to 36% of fluconazole-treated patients compared with 3% of placebo-treated patients. Fluconazole demonstrated mycologic eradication rates of 47% to 62% at the end of therapy compared with 14% for placebo. The rates at the end of follow-up were very similar, indicating that eradication of the dermatophyte was maintained over the 6-month period. All efficacy measures for the fluconazole groups were significantly superior to placebo (p = 0.0001); there were no significant differences between the fluconazole groups on these efficacy measures. The clinical relapse rate among cured patients over 6 months of follow-up was low at 4%. Fluconazole was well tolerated at all doses over the 12-month treatment period, with the incidence and severity of adverse events being similar between the fluconazole and placebo treatment groups. Mean time to clinical success in the fluconazole treatment groups was 6 to 7 months. This time frame may be used as a guideline for fluconazole treatment duration. Conclusion: The results of this study support the use of fluconazole in the treatment of distal subungual onychomycosis of the toenail caused by dermatophytes. Doses between 150 to 450 mg weekly for 6 months were clinically and mycologically effective as well as safe and well tolerated. (J Am Acad Dermatol 1998;38:S77-86.)