We have studied the plasma pharmacokinetics of idarubicin (4-demethoxy daunorubicin) in 11 elderly patients suffering from acute myeloblastic leukemia receiving orally 30 mg/m2 per day for 3 consecutive days. Idarubicin culminated in plasma 4 hr after administration and followed three similar time courses after the three administrations. Idarubicinol (13-dihydro-4-demethoxy daunorubicin) was the only fluorescent metabolite in plasma and no aglycone could be detected; idarubicinol concentration was always higher than that of unchanged idarubicin. Due to its protracted half-life (64 hr in this study), this metabolite progressively accumulated and the ratio of the areas under the curve (0-24) idarubicinol/idarubicin increased from day to day. By comparison to results obtained after i.v. administration of the drug in another study, the bioavailability of idarubicin alone can be estimated to about 21%, whereas the bioavailability of the sum idarubicin + idarubicinol is about 41%.