In an open study 31 patients undergoing bone marrow transplantation for various haematological diseases received fluconazole as prophylaxis or treatment of fungal infections. In 26 of these patients an antecedent oral prophylaxis with polyene antimycotics had failed to prevent infections with Candida species. Five of the 31 patients received fluconazole as primary prophylaxis because of non-compliance for polyene antimycotics. Fluconazole was administered orally at a daily dose of 100 mg and 200 mg, respectively (n = 29), or intravenously at a dose of 100 mg and 400 mg (n = 2). Cure or efficient prophylaxis was achieved in 22/31 patients (71%) after a median of 52 (9 to 493+) treatment days. In three patients (10%) Candida was eradicated but the infection reappeared 14-28 days after cessation of the drug; in 6 patients (20%) the infection was persistent or progressive. Four patients developed lethal Aspergillus infection while on fluconazole medication. A moderate and reversible elevation of liver function tests under therapy was observed in 9 patients and was possibly attributable to fluconazole in three of them (10%). One patient developed tremor which resolved after cessation of fluconazole. No other adverse drug reactions could be noted. We conclude that fluconazole is a relatively safe and effective drug for the prevention and treatment of superficial and, possibly, deep Candida infections in severely immunocompromised patients. However, it is presumably without preventive value in Aspergillus infections.