Clinical and cytogenetic studies were done on 8 patients with dysmyelopoietic syndrome: 6 of these patients had refractory anemia with an excess of blasts (RAEB), and 2 patients had chronic myelomonocytic leukemia (CMML) according to the French-American-British classification. The ages of these 8 patients (3 female and 5 male) ranged from 45 to 70 yr (median, 61.5 yr). Seven of the 8 patients died 3–86 mo (median, 11 mo) after the onset of symptoms of hemorrhage or infections. Cytogenetic studies of bone marrow cells with the Q-banding technique showed clonal karyotypic abnormalities in 7 of the 8 patients (87.5%). Five of the 7 chromosomally abnormal patients had very complex karyotypes; all 7 patients, however, had at least 1 of 4 specific changes: -5 (or 5Q-), -7, +8, and +21. Three of the 7 patients with abnormal karyotypes had had some exposure to potential mutagenic/carcinogenic agents. Five of the 7 patients had serial cytogenetic analyses, 4 of which showed evolution of the karyotype to further complexity; in 2 cases, this coincided with the evolution of the disease into acute leukemia. The median survival time of patients whose initial cytogenetic samples showed both normal and abnormal metaphases was more than twice that of patients who had only abnormal metaphases initially (12 mo versus 4.5 mo).