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Objective To assess the performance of the Japanese version of the Patient Health Questionnaire-9 (J-PHQ-9) for depression in primary care. Methods Participants in both phases completed the J-PHQ-9, while patients in the second phase also completed the SF-8 (the short form for the health-related QOL scale SF-36). Subjects (n = 284; male = 107, female = 177) had to return the questionnaires to their health care professional within 48 hours and undergo a diagnostic evaluation interview based on the Japanese version of M.I.N.I-Plus. Results 93 patients were diagnosed as having major depressive disorder (MDD). In the J-PHQ-9, the optimal cutpoint ≥ 10 had sensitivity of 90.5% and specificity of 76.6%. As for the categorical algorithms, the sensitivity was 80.6%; specificity was 89.5%, and a positive likelihood ratio of 7.7. The Stratum-specific likelihood ratios (SSLRs) of the J-PHQ-9 scores of 0-9, 10-14, 15-19, and 20-27 for major depression were 0.10 (95% CI: 0.05-0.20), 1.67 (95% CI: 1.02-2.76), 5.41 (95% CI: 2.87-10.22), and 11.98 (95% CI: 5.39-26.63), respectively. The relationship between the severity of J-PHQ-9 and the MCS of SF-8 was significant (χ 2 ＝ 85.72, df = 4, P ≤ 0.0001). Conclusions This study has validated the J-PHQ-9 as a useful tool for the assessment of MDD in primary care in Japan.