Objective This study aims to investigate the association of ulcerative colitis (UC) with all-cause dementia and assess differences in those with and without a total colectomy.

Design, setting and participants This Swedish prospective register-based study comprised 4.8 million individuals aged at least 59 years between 1964 and 2018 with the linkage of several Swedish national registers.

Primary and secondary outcome measures Individuals with dementia were defined according to International Classification of Diseases diagnostic codes and Anatomical Therapeutic Classification codes for medication prescriptions. Fitting Cox hazards models, the risk of developing all-cause dementia in individuals with and without UC was estimated. Further, we compared the risk of all-cause dementia among those with and without a colectomy.

Results Among 4 821 488 individuals (52.6% females) followed for 84.1 million person-years between 1964 and 2018, the incidence rate of all-cause dementia was 63.90 (63.73–64.07) events per 10 000 person-years in individuals without UC, 94.80 (92.04–97.64) among those with UC, 95.01 (92.25–97.86) in those with UC but without colectomy and 63.42 (40.92–98.31) in those with UC and a colectomy. Adjusted Cox models showed an increased all-cause dementia risk in individuals with UC (HR 1.07, 95% CI 1.04 to 1.10). We found no differences between unexposed individuals and those with UC and a colectomy (HR 0.89, 95% CI 0.57 to 1.38).

Conclusion The findings are consistent with previous evidence suggesting a slightly increased dementia risk among individuals with UC. This study provided no evidence of further risk increase of dementia among those who had a colectomy.