In this phase II multicenter trial, the efficacy and safety of mitoxantrone (Novantrone; Lederle Laboratories, Wayne, NJ) were evaluated in the treatment of 206 patients with relapsed non-Hodgkin's lymphoma (NHL) or Hodgkin's disease (HD) previously treated with other agents. Sixty-nine percent of the patients had received prior therapy with doxorubicin. The patients received 14 mg/m2 of mitoxantrone every 3 weeks. Nineteen (12%) of the NHL patients and two (7%) of the HD patients had complete responses (CRs). The combined CR and partial response (PR) rates were 37% (60 of 163) for NHL patients and 36% (10 of 28) for HD patients; the median duration of response was 323 days for NHL patients and 209 days for HD patients. The median survival times were 337 days for patients with NHL and 469 days for patients with HD. The median survival time for patients with low-grade NHL was 589 days compared with 298 days for patients with intermediate-grade NHL and 167 days for patients with high-grade NHL. The median time to treatment failure was 73 days for NHL patients and 98 days for HD patients. The major toxicity was myelosuppression, which was moderate and reversible. Nausea, vomiting, and alopecia were mild. There were two cases of congestive heart failure (CHF) considered related to treatment; both patients had received prior treatment with doxorubicin. In this group of heavily pretreated patients, mitoxantrone was effective and well tolerated. Responses were seen with mitoxantrone in patients who had relapsed after prior therapy with doxorubicin and in patients who had failed to respond to prior therapy with doxorubicin. Mitoxantrone should be evaluated in less heavily pretreated patients and should be considered for incorporation into combination chemotherapeutic regimens for the treatment of malignant lymphoma.