Objective: To examine whether performance measures improve more in accredited hospitals than in non-accredited hospital. Design and setting: A historical follow-up study was performed using process of care data from all public Danish hospitals in order to examine the development over time in performance measures according to participation in accreditation programs. Participants: All patients admitted for acute stroke, heart failure or ulcer at Danish hospitals. Intervention: Hospital accreditation by either The Joint Commission International or The Health Quality Service. Measurements: The primary outcome was a change in opportunity-based composite score and the secondary outcome was a change in all-or-none scores, both measures were based on the individual processes of care. These processes included seven processes related to stroke, six processes to heart failure, four to bleeding ulcer and four to perforated ulcer. Results: A total of 27 273 patients were included. The overall opportunity-based composite score improved for both non-accredited and accredited hospitals (13.7% [95% CI 10.6; 16.8] and 9.9% [95% 5.4; 14.4], respectively), but the improvements were significantly higher for non-accredited hospitals (absolute difference: 3.8% [95% 0.8; 8.3]). No significant differences were found at disease level. The overall all-or-none score increased significantly for non-accredited hospitals, but not for accredited hospitals. The absolute difference between improvements in the all-or-none score at non-accredited and accredited hospitals was not significant (3.2% [95% ?3.6:9.9]). Conclusions: Participating in accreditation was not associated with larger improvement in performance measures for acute stroke, heart failure or ulcer.