To determine the prevalence of activated rasoncogenes (N-ras, Harvey-ras Kirsten-ras), DNA derived from peripheral blood of 51 patients with myelodysplastic syndrome (MDS) was investigated. The method was based on the polymerase chain reaction (PCR) technique to amplify DNA, followed by restriction fragment length polymorphism (RFLP) analysis. Among the French-American-British (FAB) subtypes, N-ras mutations were found in two patients with refractory anemia with excess of blasts (RAEB), in one patient with refractory anemia with excess of blasts in transformation (RAEB-t), and in two patients with chronic myelomonocytic leukemia (CMML). MDS patients with a mutation at codon 12 of the N-ras gene showed shorter survival duration than other MDS patients of the same FAB subtypes, although these findings proved to be not statistically significant (P > 0.1). Interestingly, all but one patient with N-ras mutation developed acute myelogenous leukemia (AML). In conclusion, the presence of mutation at codon 12 of the N-ras gene might serve as a negative prognostic factor at diagnosis of MDS.