To investigate the correlation of trough imatinib mesylate (IM) levels with cytogenetic or molecular responses, we measured trough IM levels in patients with chronic myeloid leukemia, chronic phase (CML-CP), at 6 months of treatment with a standard dose of IM. Eighty-seven newly diagnosed patients with CML-CP were prospectively enrolled. Seventy-eight patients (89.7%) showed an optimal response (complete or partial cytogenetic response) at 6 months. Trough IM levels were 1378 ± 725 ng/mL. When categorized into two groups, there was a statistically significant difference in numbers of patients with optimal and suboptimal responses at 6 months (group with <1000: 80.6% vs. 19.4%; ≥ 1000: 94.6% vs. 5.4%; p = 0.032), and in numbers of patients with early major molecular response (early-MMR) and without MMR at 6 months (group with <1000: 3.2% vs. 96.8%; ≥ 1000: 21.4% vs. 78.6%; p = 0.047). In conclusion, the incidence of optimal cytogenetic response or early-MMR in patients with CML-CP treated with IM for 6 months was significantly higher in those with a trough level of ≥ 1000 compared with those with a level of <1000. Dose escalation of IM can be one option in patients with CML showing suboptimal response or resistance to the standard dose of IM, especially with low trough plasma IM levels (<1000 ng/mL).