A randomized, double-blind, placebo-controlled trial assessed the efficacy and toxicity of 400 mg/day fluconazole in preventing fungal infections during the first 75 days after marrow transplantation. During prophylaxis, systemic fungal infections occurred in 10 (7%) of 152 fluconazole-treated patients compared with 26 (18%) of 148 placebo-treated patients (P = .004). There were no Candida albicans infections in fluconazole recipients compared with 18 in placebo recipients (P < .001) and no significant increase in Candida infections other than C. albicans. Fluconazole also significantly reduced the incidence of superficial fungal infections (P < .001), fungal colonization (P = .037), and empiric amphotericin B use (P = .005). The probability of survival was improved in fluconazole recipients, in whom 31 deaths occurred up to day 110 after transplantation compared with 52 deaths in placebo recipients (P = .004). No clinically significant toxicity was detected with fluconazole use. Prophylactic fluconazole was safe and significantly reduced systemic fungal infections with other benefits, including improved survival at day 110 after marrow transplantation.