Idarubicin (4-demethoxydaunorubicin) is an orally active anthracycline. We treated 26 patients with 37 courses of the drug on a schedule of oral administration weekly × 3 followed by a 3-week rest period. The maximum tolerated dose on this schedule was 22.5 mg/m2 weekly × 3 every 6 weeks, with consistent myelosuppression being the dose-limiting toxicity; other toxicity was minimal. Pharmacologic studies showed a mean alpha half-life of 1.6 ± 0.3 h and a beta half-life of 39 ± 8.4 h for idarubicin. This schedule was well tolerated, with consistent toxicity patterns seen. Pharmacologic studies confirmed prolonged exposure to the drug and its active metabolite. In comparison with other schedules, this one may offer advantages in terms of consistent hematologic toxicity and prolonged exposure to both the parent compound and its active metabolite. Dose intensity was comparable with that on other schedules.