Thirty one patients with solid tumors were entered into a phase I trial with idarubicin, a new anthracycline antibiotic with oral antitumor activity in animals. The drug was scheduled to be given for 4 consecutive weeks at doses ranging from 10 to 20 mg/m2. Leukopenia was the dose-limiting toxicity. Thrombocytopenia was occasionally seen. Since several patients could not receive the third and fourth administrations of the drug at 17.5 and 20 mg/m2, higher doses were administered only for 2 consecutive weeks. With this schedule, the maximum tolerated dose was 25 mg/m2 and leukopenia was again the dose-limiting toxicity. With both schedules, myelosuppression was highly variable and could not be related to prior therapy, bone or liver metastases, or performance status. Other toxicities were mild to moderate and were dominated by nausea and vomiting which were observed in 29% of the patients. Alopecia and mucositis were unfrequent and cardiac toxicity was not observed. Starting doses of 15 mg/m2 for 4 consecutive weeks or 20 mg/m2 for 2 consecutive weeks could be proposed for oral phase II studies with idarubicin, under careful pharmacokinetic monitoring.