Cytogenetic studies were performed in 120 patients with de novo myelodysplastic syndrome (MDS) classified according to FAB criteria. Twenty-eight patients had refractory anemia (RA), 14 had refractory anemia with ring sideroblasts (RARS), 45 had refractory anemia with blast excess (RAEB), 19 had refractory anemia with blast excess in transformation (RAEB-t), and 14 had chronic myelomonocytic leukemia (CMMoL). Fifty patients (42%) had clonal chromosome anomalies at initial analysis. The most common cytogenetic anomalies were: 5q- (11 patients), trisomy 8 (nine patients), -7/7q- (6 patients), 12p- (five patients), followed by structural anomalies of chromosome 17 (four patients), and loss of Y chromosome (three patients). The prognostic value of chromosome anomalies was examined by comparison of the significance of single chromosome anomalies (34 patients) versus multiple cytogenetic changes (16 patients). Patients with multiple anomalies had a shorter survival (8 months) than patients with single anomalies (18 months) or those with a normal karyotype (36 months). All these differences were significant. The incidence of multiple anomalies was higher in patients with RAEB and RAEB-t than in those with RA, RARS, and CMMOL (p less than 0.05). However, no chromosome anomaly was specifically associated with any group of FAB classification. Transformation to acute leukemia was observed in 25% of patients with normal karyotype, 41% of patients with single anomalies, and 50% of patients with multiple changes. The incidence of leukemic transformation was significantly higher in patients with multiple anomalies than in those with a normal karyotype (p less than 0.05). Thus, in the present study, FAB classification and chromosome anomalies were of independent prognostic significance. Sequential cytogenetic studies were performed in 23 patients to correlate the cytogenetic and clinical findings during the course of the disease. Six of seven patients with transformation to acute leukemia showed a karyotypic evolution. These findings agree with the view that an unstable karyotype can be associated with a poor prognosis.