The marriage of cytogenetics and molecular biology has resulted in major advances in our understanding of acute nonlymphocytic leukemia. These technologies reveal a number of clearly recognizable syndromes of acute nonlymphocytic leukemia. This review describes the salient features of several of these syndromes: acute myelomonocytic leukemia with abnormal eosinophils; acute nonlymphocytic leukemia with 11q abnormalities [biphenotypic leukemia with t(4;11); and acute monocytic leukemia with t(9;11)]; acute nonlymphocytic leukemia with t(8;21); acute promyelocytic leukemia; acute nonlymphocytic leukemia with normal or elevated platelet counts and rearranged 3q21 and 3q26; and acute nonlymphocytic leukemia with increased basophils and t(6;9). The pathogenesis of therapy-related leukemias is discussed also.