Intensive antileukemic treatment was evaluated in 22 patients with secondary acute myelogenous leukemia (sAML) and 14 patients with myelodysplastic syndrome (MDS). Results of combination remission-induction chemotherapy were compared with 126 patients contemporarily treated for primary AML. The duration of hypoplasia, induced by remission induction chemotherapy, tended to be longer in the sAML and MDS patients when compared to de novo AML, but reached significance only for the duration of thrombocytopenia: 26 days versus 18 days (P less than 0.01). The number of hypoplastic deaths during remission-induction chemotherapy of patients with sAML and MDS was low. Four of the 36 patients treated for sAML or MDS died during hypoplastic phases induced by remission-induction chemotherapy. The complete remission (CR) rates were similar in primary AML (67%), sAML (62%), and MDS (64%). The CR rates of patients younger than 45 years were 75% for de novo AML, 75% for sAML, and 71% for MDS. Remission rates in patients older than 45 years were identical in the three subgroups but significantly (P less than 0.005) inferior to those obtained in younger patients: 56%, 50%, and 57%, in de novo AML, sAML, and MDS, respectively. The remission duration without bone marrow transplant (BMT) was significantly shorter (P less than 0.01) in MDS and sAML when compared with primary AML. Long-lasting CR in MDS and sAML was only obtained in three of the six patients treated with allogeneic BMT. Intensive antileukemic therapy could be considered in young patients with MDS and life-threatening cytopenias or patients with sAML.