Eight patients with small cell bronchogenic carcinoma treated with intensive combination chemotherapy, with and without radiotherapy, have been followed for a minimum of two and a half years without relapse. One patient, after a prodrome of macrocytic sideroblastic anemia, leukopenia, and thrombocytopenia, experienced erythroleukemia 34 months after starting chemotherapy, and cytogenetic studies revealed extensive chromosomal abnormalities. Another patient had persistent macrocytic anemia and pancytopenia two years after cessation of therapy. The remaining six patients had normal peripheral blood smears and cell counts. A significant incidence of preleukemia syndromes and acute leukemia appearing as late complications in intensively treated small cell lung cancer patients requires confirmation in larger series of long-term survivors. Prospective determination of marrow karyotype abnormalities may help to identify patients at greatest risk for developing secondary leukemia.