Idarubicin, a new analogue of daunorubicin, was administered i.v. at a dose of 15 mg/m2 to 31 previously treated patients with unfavorable non-Hodgkin's lymphoma. Clinical characteristics included median age, 69 years; performance status, 1; and prior chemotherapeutic regimens, 1. Twenty of the patients were relapsing after prior therapy and 11 were refractory; 29 had received prior anthracycline or anthracenedione. Responses were observed in 43% of patient (3 complete remission and 10 partial remission) with a median duration of 10+ months (2-29+ months). Idarubicin was well tolerated with nonhematological toxicities (nausea/vomiting, mucositis, and anorexia) seen in less than 50% of patients. Median hematological values during the first cycle for this dosage included WBC, 1,300/mm3; platelets, 129,000/mm3; and hemoglobin, 10.9 mg/dl. With dose escalation, hematological toxicity was dose limiting. Symptomatic cardiac toxicity was observed in one patient who had received maximum dose doxorubicin and radiotherapy. Median values for the cardiac ejection fraction during the full course of therapy for the entire group of patients were 0.62 (initial) and 0.60 (final). Idarubicin in i.v. form is an active drug in previously treated patients with unfavorable non-Hodgkin's lymphoma.