Activation of the N- and K-ras proto-oncogenes is the most common molecular abnormality in myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). In retrospective studies, approximately 3-36% of MDS patients were reported to harbor a mutated ras proto-oncogene, with some series suggesting the presence of ras-mutations are associated with progressive disease and a poor prognosis. Since hematopoietic growth factors such as granulocyte colony-stimulating factor (G-CSF) are currently used for therapy in MDS but may stimulate the proliferation of leukemic cells, we assessed the frequency and significance of ras mutations in 27 MDS patients, 15 of whom underwent G-CSF therapy. Patients were analyzed for the presence of mutations in codons 12, 13, and 61 of the N- and K-ras proto-oncogenes. Only three patients (11%, two refractory anemia with excess of blasts (RAEB), one RAEB in transformation (RAEB-T)) harbored activated ras oncogenes with the mutations localized in N-ras codons 12 and 61. Patients were followed for periods of up to 4 years or until death supervened. Patients exhibiting ras mutations were no more likely to develop AML compared to ras-negative patients (1/3 vs. 10/24) or to have decreased survival (p = 0.64). These data indicate that, in this group of MDS patients, ras mutations do not appear to correlate with a poor prognosis, and do not adversely interact with exogenously administered G-CSF.