The response criteria proposed by European Leukemia Net are useful to predict the prognosis of de novo chronic myeloid leukemia (CML) patients in the chronic phase (CP) treated with imatinib. However, the clinical significance of late suboptimal response, which is defined as the achievement of CCgR without MMR after 18 months, is controversial. In this study, we retrospectively analyzed the clinical courses of 16 CML-CP patients, who satisfied the criteria for late suboptimal response. The median duration of imatinib treatment was 62 (25∼87) months. The median starting dose of imatinib was 400 mg/day. Imatinib dose was escalated to 600∼800 mg/day in 10 patients for various reasons. Among 4 patients who continued high-dose imatinib for late suboptimal response, 2 patients subsequently achieved MMR, and BCR-ABL mRNA transcript levels were decreasing in 2 patients. However, imatinib was kept at 300 or 400 mg/day in 6 patients. Among these six patients, 4 patients achieved MMR, while 2 failed to achieve MMR. None of 16 patients progressed to the acute phase or blast phase. Imatinib dose escalation was effective for late suboptimal response. Furthermore, a second tyrosine kinase inhibitor such as nilotinib may be more potent to reduce the risk of disease progression by achieving earlier MMR.