We present four patients with acute nonlymphocytic leukemia, one M2 and three M4 French-American-British (FAB) types and one patient with refractory anemia with excess of blasts in transformation who at diagnosis had trisomy of chromosome 4 as the primary karyotypic anomaly. This chromosome anomaly probably defines a previously undescribed subset of acute nonlymphoid leukemias. Hematologic characteristics commonly found in these patients were dysplastic features of all bone marrow lineages, suggesting that trisomy 4-associated disorders involve the early myeloid hematopoietic stem cell.