Clinical, cytomorphologic, and cytogenetic investigations were carried out in a series of 76 secondary MDS and ANLL. Chromosome abnormalities were more frequent in patients with a history of multiple myeloma or macroglobulinemia (92%) and myeloproliferative disorders (82%) than in patients with previous breast cancer (40%). The secondary hematologic malignancies were mostly a trilineage bone marrow disorder. The most commonly found cytogenetic anomaly was monosomy 7, followed by total or partial loss of chromosome 5. In addition six other chromosomes, i.e., chromosome 3, 8, 9, 12, 17, and 21 seemed to be consistently involved in the pathogenetic mechanisms of secondary leukemia and MDS.