Objective: The objective was to reveal the relationship between dose and concentration of atomoxetine. Method: Fifty-five blood samples of 33 patients with ADHD were examined using high-performance liquid chromatography. Results: The plasma concentrations were 53.2 ± 67.0, 298.0 ± 390.5, and 639.3 ± 831.9 ng/mL at doses of 40 mg, 80 mg, and 120 mg, and the concentration/dose were 1.33 ± 1.67, 3.73 ± 4.88, and 5.33 ± 6.93 ng/mL/mg, respectively. Statistical analyses revealed a significant correlation between the concentration and the dose of atomoxetine (p = .004), and a trending toward significance in the difference in the concentration/dose in the three dosage groups (p = .064). The concentration/dose at 40 and 80 + 120 mg/day were 1.33 ± 1.67 and 4.22 ± 5.53 ng/mL/mg, the latter was significantly higher than the former (p = .006), which suggested non-linear pharmacokinetics. Conclusion: Clinicians should carefully titrate in high dose atomoxetine treatment.