BackgroundThe Constant-Murley Score (CMS) is a relatively unique shoulder assessment tool because it combines patient-reported outcomes (pain and activity), performance measurement and clinician-reported outcomes (strength and mobility). With these characteristics, the effect of patient-related psychological factors on the CMS remains debated. We aimed to investigate which parameters of the CMS are influenced by psychological factors by assessing the CMS before and after rehabilitation for chronic shoulder pain.MethodsThis retrospective study screened all patients (18–65 years old) who were admitted for interdisciplinary rehabilitation for chronic shoulder pain (≥ 3 months) between May 2012 and December 2017. Patients with unilateral shoulder injuries were eligible. Exclusion criteria were shoulder instability, concomitant neurological injuries, complex regional pain syndrome (including Steinbrocker syndrome), heavy psychiatric issues, and missing data. The Hospital Anxiety and Depression Scale, Pain Catastrophizing scale, and Tampa Scale of Kinesiophobia were administered before and after treatment. Regression models were used to estimate associations between psychological factors and the CMS.ResultsWe included 433 patients (88% male, mean age 47±11 years) with a median duration of symptoms of 392.2 days (interquartile range: 266.5-583.5). Rotator cuff issue was present in 71% of patients. During interdisciplinary rehabilitation, patients were followed for a mean of 33.6±7.5 days. The mean CMS at entry was 42.8 ±15.5. The mean gain in CMS after treatment was 10.6 ±10.9. Before treatment, psychological factors were significantly associated with only the pain CMS parameter: -0.37 (95% CI: -0.46 to -0.28), p <0.001. After treatment, psychological factors were associated with the evolution of the four CMS parameters: -0.12 (-0.23 to -0.01) to -0.26 (95% CI: -0.36 to -0.16), p<0.05.ConclusionsThis study raises the question of a distinct assessment of pain when assessing shoulder function with CMS in patients with chronic shoulder pain. The separation of the “pain parameter” from the overall CMS score seems illusory with this tool that is used worldwide. However, clinicians should be aware that psychological factors can negatively influence the evolution of all CMS parameters during follow-up, which argues for a biopsychosocial approach to patients with chronic shoulder pain.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12891-023-06441-7.