Background: Onychomycosis is a prevalent infection of the nail caused primarily by dermatophytes. Fluconazole is active in vitro against the most common pathogens, penetrates into the nail bed, and is clinically effective in the treatment of a wide variety of fungal infections. Objective: The purpose of this study was to assess the safety and efficacy of oral fluconazole 150, 300, and 450 mg administered once weekly compared with placebo in the treatment of distal subungual onychomycosis of the fingernail caused by dermatophytes. Methods: This was a multicenter, randomized, double-blind, placebo-controlled study enrolling 349 patients with onychomycosis of the fingernails. Clinical and mycologic efficacy as well as measures of safety were assessed monthly for a maximum of 9 months of treatment, with additional safety visits occurring at weeks 2 and 6. For inclusion, patients were required to have clinically and mycologically documented onychomycosis of the fingernail caused by dermatophytes with at least 25% involvement of the target fingernail. After end of therapy, patients with improved or cured fingernails entered a blinded 6-month follow-up without drug treatment during which efficacy was assessed every 2 months. Efficacy was assessed by clinical (visual) and mycologic (microscopic and culture) measures. Clinical measures included assessments of the percentage of target nail involvement, measurement of the distance from the nail fold to the proximal onychomycotic border, and signs and symptoms of onychomycosis. Results: Fluconazole was significantly superior to placebo in eradicating clinical and mycologic symptoms of onychomycosis, both at the end of active treatment and at 6 months after treatment ( p = 0.0001 for all efficacy measures). At the end of therapy, 91% to 100% of patients in the fluconazole groups were judged clinical successes, defined as reduction of the affected area of the target nail to less than 25% or cure, compared with 8% for placebo. Clinical cure rates at end of therapy were 76%, 85%, and 90% for fluconazole 150, 300, and 450 mg, respectively, compared with 3% for placebo. These clinical success and cure rates were largely maintained or improved during follow-up. Clinical relapse in cured patients during the follow-up period was very low (1.5% to 3.3%). Fluconazole demonstrated mycologic eradication rates of 89% to 100% at the end of treatment and 90% to 99% at the end of follow-up; for placebo the rates were 8% and 12%, respectively. Conclusion: Fluconazole administered once weekly is safe and effective in eradicating distal subungual onychomycosis of the fingernail caused by dermatophytes. (J Am Acad Dermatol 1998;38:S87-94.)