Idarubicin, a new analogue of daunorubicin, was administered intravenously at a dose of 15 mg/m2 to 31 patients with 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 CR and 10 PR) with a median duration of 10 + months (2-29+ months). Idarubicin was well tolerated with non-hematologic toxicities (nausea/vomiting, mucositis, and anorexia) seen in <50% of patients. Median hematologic values during the first cycle for this dosage included WBC 1300/mm3 platelets 129,000/mm3, and hemoglobin 10.9 mg/dl. With dose escalation, hematologic 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 intravenous form is an active drug in previously treated patients with unfavorable non-Hodgkin's lymphoma. Further studies employing idarubicin in non-Hodgkin's lymphoma should be considered. Cardiac function should be followed in trials utilizing anthracycline-type chemotherapeutic agents.