A randomized, double-blind trial comparing oral fluconazole (400 mg daily) with placebo as prophylaxis for adult patients receiving intensive cytotoxic therapy for acute leukemia or autologous bone marrow transplantation was conducted in 14 Canadian university-affiliated hospitals. Although fluconazole prophylaxis did not obviate the need for parenteral antifungal therapy compared with placebo (81 [57%] of 141 vs. 67 [50%] of 133, respectively), its use resulted in fewer superficial fungal infections (10 [7%] of 141 vs. 23 [18%] of 131, respectively; P = .02) and fewer definite and probable invasive fungal infections (9 vs. 32, respectively; P = .0001). Fluconazole recipients had fewer deaths attributable to definite invasive fungal infection (1 of 15 vs. 6 of 15, respectively; P = .04) and achieved more frequent success without fungal colonization (52 [37%] of 141 vs. 27 [20%] of 133, respectively; P = .004; relative risk reduction, 85%) than did placebo recipients. Patients benefiting the most from fluconazole prophylaxis included those with acute myeloid leukemia who were undergoing induction therapy with cytarabine plus anthracycline-based regimens and those receiving marrow autografts not supported with hematopoietic growth factors. Fluconazole prophylaxis reduces the incidence of superficial fungal infection and invasive fungal infection and fungal infection-related mortality among patients who are receiving intensive cytotoxic chemotherapy for remission induction.