The human organic cation transporter 1 (hOCT1) is the major active influx protein responsible for the transport of imatinib mesylate (IM) into cells. Previous studies have used ¹⁴C-labeled IM to demonstrate a link between chronic myeloid leukemia (CML) molecular response and hOCT1 activity. However, this method is not convenient in routine clinical practice. Hence, we detected hOCT1 protein expression levels (C_hoct1) of peripheral blood in CML patients and evaluated the relationship between C_hoct1 and IM response. A total of 83 patients who were diagnosed with Philadelphia chromosome (Ph)-positive CML with IM therapy and 31 heathy donors were collected. C_hoct1 were detected by indirect immunofluorescent flow cytometry. The study showed that C_hoct1 expression was higher in healthy donors than in CML patients (n = 31, 9.11 ± 6.04; n = 35, 5.60 ± 3.74; p = 0.005). Both C_hoct1 and plasma IM trough concentration (C_min, n = 83) were significantly higher in patients with major molecular response (MMR) than those without (p = 0.011; p = 0.001, respectively), and patients with C_hoct1 ≥4.745 and C_min ≥1,385 ng/ml were more likely to achieve MMR. hOCT1 expression levels measured using flow cytometry is a convenient and clinically available technique. The hOCT1 expression level can be an important predictor in CML patients treated with IM. © 2014 S. Karger AG, Basel