Intensive antileukemia treatment was evaluated in 22 patients with untreated secondary acute myelogenous leukemia (sAML) and 14 patients with bad prognosis myelodysplastic syndrome (MDS). Results of combination remission-induction chemotherapy were compared with 126 patients treated for primary AML. The duration of hypoplasia 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 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. 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 subsequent hypoplastic phases induced by remission-induction chemotherapy. The remission duration without bone marrow transplantation (BMT) was significantly shorter (p less than 0.03) in MDS and sAML, when compared with primary AML. Longlasting complete remissions in MDS and sAML were only obtained in 3 of the 6 patients treated with allogeneic BMT. Intensive antileukemic therapy should be considered in young patients with MDS and life-threatening cytopenias or patients with sAML or RAEBt.