Mitoxantrone, an anthracenedione, has been shown to be as effective as doxorubicin, but with less local or systemic toxicity, when used for the treatment of advanced breast cancer. As the high molecular weight and hydrophilic property of this drug let us predict a slow intracavity resorption, we tested its use in the treatment of malignant effusions refractory to systemic chemotherapy. 18 women, 43 to 83 years old, with cytologically demonstrated metastatic pleural effusions, and with prominent clinical symptoms, were included in the study. All these patients were refractory to hormonotherapy and combination chemotherapy (previous regimens included anthracyclines in 17 patients but none had received systemic mitoxantrone). No previous local treatment had been attempted. During the study period, no other treatment has been given. Mitoxantrone (6 mg/m2) has been administrated after effusion aspiration. A complete response was seen in 8 patients, a partial response in 5, and no change in the 5 others but one had received only one injection on account of a transitory shock immediately after. No others side effects were reported (fever, local pain, alopecia, vomiting). No patient had evidence of myelosuppression. These results suggest that intracavity injection of mitoxantrone is feasible and generally safe in most patients. Some efficiency has been seen in previously heavily treated patients refractory to systemic chemotherapy. Local administration of mitoxantrone deserves further investigation.