Background and purpose — Although most patients have good outcomes after shoulder arthroplasty for osteo-arthritis, certain risk factors may lead to disappointing out-comes. We assessed risk factors for a worse outcome after anatomical shoulder arthroplasty for osteoarthritis. Our hypothesis was that previous surgery for instability would be a risk factor for a worse outcome independent of age, sex, and arthroplasty type. Patients and methods — We included arthroplas-ties reported to the Danish Shoulder Arthroplasty Registry between 2006 and 2018 (n = 3,743). The Western Ontario Osteoarthritis of the Shoulder (WOOS) index at 1 year was used as outcome. The total score was converted to a percentage of a maximum score. The general linear model was used to analyze differences in WOOS. Age, sex, arthroplasty type, and previous surgery were included in the multivariate model. Estimates were given with 95% confidence intervals (CI). Results — The mean WOOS score was 78 for patients with no previous surgery and 55 for patients with surgery for instability. The mean difference was –16 (CI –10 to –22) in the multivariate model. Hemiarthroplasty had a worse outcome compared with total shoulder arthroplasty and young patients had a worse outcome compared with older patients. The mean differences in the multivariate model were –12 (CI –10 to –14) and –11 (CI –8 to –14) respectively. Interpretation — Patients with previous surgery for instability had worse results independent of age, sex, and arthroplasty type and should be informed about their indi-vidual risk of a worse outcome.