Background: Mutations in ras geneshave been observed in a variety of cancersand were found to play an important role in human leukemogenesis and in preleukemic disease as myelodysplastic syndrome (MDS). The purpose of this study was to determine the prevalence of mutated K-ras oncogene in  myelodysplastic syndrome (MDS); with a special emphasis on their possible role in affecting clinical status, relation to karyotypic pattern; response to therapeutic measures; its impact on the fate of the disease and overall survival. Subjects & methods: Detection of point mutation  of  Kirsten-ras (K-ras) gene in 30 patients suffering  from myelodysplastic syndrome was carried out  using quantitative enriched polymerase chain reaction (QEPCR) and was confirmed by sequencing. QEPCR is a two- stage PCR procedure with modified primers that enriches mutant alleles, via restriction endonuclease digestion of normal alleles and enables identification of one mutant allele among 100,000 normal alleles.  Results: Activating mutations of the codon 12 of K-ras gene were detected in 7/30 (23.3%)cases of MDS, the most  common mutation involved a substitution of aspartic acid forglycine (GGT→GAT). The incidence of K-ras mutations was found to be significantly associated with refractory anemia with excess blasts type II (RAEBII) and unclassified (UC) MDS than other subtypes (p=0.005), and was significantly associated with hypercellular bone marrow (p=0.04) showing marked dyserythropoitic changes. Furthermore, mutant K-ras gene was found to be significantly associated with abnormal karyotypes (p=0.04). Patients with mutated K-ras gene were significantly associated with either high or intermediate risk according to International Prognostic Scoring System (IPSS) (p=0.001). 6/7(85.7%) of those carrying the mutation showed poor response to treatment compared to non carriers with a statistical significant difference (p=0.009). Five out of eight (62.5%) patients who were transformed to AML carried the mutant K-ras gene, their subtypes were RAEBІІ and unclassified MDS with abnormal cytogenetics mainly Monosomy 7. Overall survival was detected using Kaplan-Meiercurve and the mean survival time of patients who carried K-ras mutations were significantly lower than those without the mutation (Log rank test=12.7; p=0.0004). Conclusion: MDS patients bearing an mutated K-ras oncogene frequently showed poor response to treatment; leukemic progression of the disease and shorter overall survival, suggesting that an activated K-ras oncogene is a critical factor for prognostic evaluation; therapeutic decision and monitoring of response to treatment of MDS patients.