Twenty-one adult patients with refractory solid tumors were treated on a phase I study of idarubicin (4-demethoxydaunorubicin) administered daily for 3 days every 3 weeks. Nineteen of the patients had received previous chemotherapy (including 13 with prior anthracyclines), and 12 had received prior radiotherapy. Idarubicin dose levels of 10, 15, 17.5, 20, and 25 mg/m2 were explored. Hematological toxicity was dose-related. Other toxicity was acceptable. Only one patient (treated with an idarubicin dose of 17.5 mg/m2/day) developed neutropenic fever, from which he recovered. Further dose escalations beyond 25 mg/m2 were not carried out because of the increasing length of time required for recovery from granulocytopenia at higher doses. No patient experienced a major response, but minor responses were seen in 3 patients with carcinomas of the colon, breast, and kidney respectively. Further phase II studies of oral idarubicin at a starting dose of 20-25 mg/m2 daily times 3 days in patients with good bone marrow reserves are recommended. Because of the degree of neutropenia expected, patients would have to be observed carefully.