One hundred eighty-eight unselected consecutive patients with “de novo” myelodysplastic syndrome (MDS) were studied cytogenetically. They were subclassified as 4 refractory anemia with ringed sideroblasts (RARS), 67 refractory anemia (RA), 58 refractory anemia with excess of blasts (RAEB), 40 RAEB in transformation (RAEB-t), and 19 chronic myelomonocytic leukemia (CMML). The overall incidence of chromosome abnormalities was 69%. The RAEB and RAEB-t patients showed karyotypic changes, more often than RA and CMML (76% and 100% vs. 56% and 42%, respectively). The most frequent single anomaly was del(5)(q13-q22q33) (22 cases), followed by monosomy 7 or del 7q (11 cases), del(11) (q14q23) (8 cases), trisomy 8 (4 cases). Complex karyotypes (defined by the presence of three or more structural or numerical abnormalities) were detected in 33 patients. With regard to the FAB classification, del (5)(q13q33) was associated with RA, and complex rearrangements with RAEB and RAEB-t. Leukemic transformation occurred in 66 patients (46%), none with a normal karyotype or del(11)(q14q23) as single abnormality. In patients carrying 5q- alone, acute evolution correlated with proximal breakpoint localization, being found in no case with del(5)(q13q33) but in three out of four cases with del(5)(q22q33). Acute leukemia (AL) progression happened in all cases with complex rearrangements and monosomy 7 or del(7q). Two of the four trisomy eight patients evolved in AL. By using the Cox proportional hazard regression analysis it was demonstrated that the karyotype abnormality was a significant predictor of leukemic transformation (P < 0.001). Patients with abnormal karyotypes without complex abnormalities had a survival (median survival 12 months) shorter than that of cases with only normal metaphases (median 83 months) (P < 0.001); patients with a mixture of normal/abnormal metaphases had a median survival of 31 months. The median survival for complex karyotypes was 7 months. Among cases with single defects, del(5)(q13q33) showed the best survival (64 months), mono- somy 7 and del (7q) the worst (7 months) (P < 0.001). © 1994 Wiley-Liss, Inc.