BACKGROUND: The Common Mental Disorders Questionnaire (CMDQ) is used to assess patients' mental health. It has previously been shown to provide a sensitive and specific instrument for general practitioner setting but has so far not been tested in hospital setting or for changes over time (test-retest). The aim of this study is, by means of a test-retest method, to investigate the reliability of the instrument over time with total hip replacement (THR) patients. METHODS: Forty-nine hip osteoarthritis patients who had undergone THR answered the questionnaire twelve months after their operation. Fourteen days later they completed it again. Covering emotional disorder, anxiety, depression, concern, somatoform disorder and alcohol abuse, the questionnaire consists of 38 items with six subscales, each of which has between 4 to 12 items. A five-point Likert scale (from 0-4) is used. RESULTS: For each of the 38 questions, a quadratic-weighted Kappa coefficient of 0.42 (0.68 - 0.16) to 0.98 (1.00 - 0.70) was found. A Cronbach's alpha of 0.94 for all the questions indicated high internal consistency. CONCLUSION: The results showed a moderate to almost perfect reliability of CMDQ of this specific population. TRIAL REGISTRATION: Current Controlled Trials: NCT01205295.