The effect of a new azole antifungal drug, fluconazole, on cyclosporin concentration and pharmacokinetics was studied in ten bone marrow transplant recipients. There were no significant statistical or clinical differences in mean cyclosporin trough concentrations, peak concentrations and AUCs after fluconazole given either as a single 100 mg oral dose or as 100 mg daily for 14 days. Fluconazole was well tolerated without drug related side effects. Slight to moderate increases of liver laboratory parameters were observed in four patients during the multiple dose sequence, but these changes might be attributable, in part, to the underlying diseases or concomitant therapies. The data suggest a lack of clinically relevant pharmacokinetic interaction of fluconazole with cyclosporin. It is concluded that fluconazole may be administered to cyclosporin treated patients without enhanced risk of toxicity.