We retrospectively analyzed the clinical outcome of chronic myelogenous leukemia (CML) patients who received imatinib mesylate (IM) as initial therapy at Tokyo University Hospital. In all patients, molecular response as well as hematological and cytogenetic response was measured routinely. Our cohort showed respectable outcome with complete cytogenetic response (CR) of 82% in 1 year, and 97% in 5 years. The estimated progression free survival of 29 patients who were in chronic phase at diagnosis was 97% and the estimated OS was 92.4% at 5 years. Although IM interruption due to side effects was observed in about half of the cases, the discontinuation was only temporary in all cases. However, patients who showed persistent intolerance with adequate dose of IM (300 mg/day or more) showed poor molecular response.