Chronic myeloid leukemia (CML) is a triphasic clonal myeloproliferative disorder characterized by the presence of Philadelphia chromosome (Ph) in over 95% of cases. The Ph chromosome results from a reciprocal translocation between the long arms of chromosomes 9 and 22. The translocation produces the BCR-ABL chimeric gene that codes for an abnormal tyrosine kinase protein,and malignant proliferation of myeloid cells. Imatinib mesylate (Glivec), a selective inhibitor of this kinase, is now widely used as the first choice of therapy in patients with CML. The objective was to determine the effect of Imatinib on the survival of different risk groups based on the Sokal and Hasford scoring systems. The study period was between August 2003 and December 2009. It was both retrospective and prospective and included all confirmed cases of CML managed at the OAUTHC Ile-Ife within the study period. One hundred and eighty patients, 110 males (61%) and 70 females (39%), age ranging from 11 to 75 years (median = 36 years), who had been on imatinib therapy for at least 3 months were recruited into the study. Patients were followed up with complete blood count and differentials on days 1, 30, 90 and thereafter at three-monthly. Repeat Karyotyping was carried out 6 and 12 months into therapy and then yearly thereafter. Overall survival (OS), progression-free survival (PFS) and frequency of complete cytogenetic remission (CCR) were evaluated. All parameters were analyzed using the Kaplan- Meier technique and p-values < 0.05 were considered significant. Overall survival was 98% and 92%, while PFS was 89% and 81%, at one and two years respectively. Both the Sokal and Hasford risk groups were sufficiently predictive of differences in PFS (p = 0.002 and p < 0.001 respectively). However, neither of the scoring systems was sufficiently predictive for differences in OS or CCR. The results from this study has shown that the Sokal and Hasford scoring systems are inadequate in predicting OS and CCR of CML patients managed on imatinib, as compared to their usefulness with previous chemotherapeutic agents.