Nineteen patients (range 27–79 years) with MDS (n = 14) or subsequent ANLL (n = 5) were treated with low doses (10 mg/m2/24 h for 10 days) of aclacinomycin (ACM). An intensified second schedule was administered to four patients. Of the 19 patients five were pretreated with low-dose cytosine arabinoside (ARA-C) or the TAD-9 schedule. Diagnosis was established by bone marrow cytology and routine hematologic procedures. Cytogenetic analysis was possible in 14 of 19 patients. Therapy was instituted because of progressive anemia and/or thrombocytopenia in all cases and performed by continuous intravenous infusion. According to CALGB criteria complete and partial remissions were achieved in 14% and 36% of the MDS patients, whereas no remissions could be obtained in secondary ANLL. Patients with complete remission (CR) as well as those with partial remission (PR) showed significantly prolonged median survival times as compared to those with no change (NC) or progressive disease (PD): 22 months for responders, 3 months for nonresponders. Censored median survival was 22 months for RAEB, 14 months for RAEB-T, but only 2 months for secondary ANLL. Evaluation of risk factors gave evidence of a poorer prognosis for patients with advanced initial thrombocytopenia (