Objective: To assess the psychometric properties of the Patient Health Questionnaire-9 (PHQ-9) as a screening instrument for depression in elderly patients with diabetes mellitus (DM) and chronic obstructive pulmonary disease (COPD) without known depression. Study Design and Setting: DM and COPD patients aged >59 years were selected from general practices. A test-retest was conducted in 105 patients. Criterion validity, using the Mini International Neuropsychiatric Interview psychiatric interview to diagnose major depressive disorder (MDD) and any depressive disorder (ADD) as diagnostic standard, was evaluated for both summed and algorithm-based PHQ-9 score in 713 patients. Correlations with quality of life and severity of illness were calculated to assess construct validity. Results: Cohen's kappa for the algorithm-based score was 0.71 for MDD and 0.69 for ADD. Correlation for test-retest assessment of the summed score was 0.91. The algorithm-based score had low sensitivity and high specificity, but both sensitivity and specificity were high for the optimal cut-off point of 6 on the summed score for ADD (Se 95.6%, Sp 81.0%). Correlations between summed score and quality of life and severity of illness were acceptable. Conclusion: The summed PHQ-9 score seems a valid and reliable screening instrument for depression in elderly primary care patients with DM and COPD. © 2008 Elsevier Inc. All rights reserved.