Background & Aims: We aimed to discern the relative risk for several chronic inflammatory conditions in patients with ulcerative colitis (UC) and Crohn s disease. Methods: We used the population- based University of Manitoba IBD- Database that includes longitudinal files on all patients from all health system contacts identified by International Classification of Diseases, 9th revision, Clinical Modification codes for visit diagnosis. From the provincial database we extracted a control cohort matching the IBD patients 10 ∶ 1 by age, sex, and geography. We considered a potential comorbid disease to be present if the patient had 5 or more health system contacts for that diagnosis. The comorbid disease period prevalence was analyzed separately for patients with UC and Crohn s disease and a prevalence ratio was calculated comparing the IBD populations with the matched cohort. Results: There were 8072 cases of IBD from 1984 to 2003, including UC (n = 3879) and Crohn s disease (n = 4193). There was a mean of approximately 16 person- years of coverage for both patients and control patients. Both UC and Crohn s disease patients had a significantly greater likelihood of having arthritis, asthma, bronchitis, psoriasis, and pericarditis than population controls. An increased risk for chronic renal disease and multiple sclerosis was noted in UC but not Crohn s disease patients. The most common nonintestinal comorbidities identified were arthritis and asthma. Conclusions: The finding of asthma as the most common comorbidity increased in Crohn s disease patients compared with the general population is novel. These may be diseases with common causes or complications of one disease that lead to the presentation with another. Studies such as this should encourage further research into the common triggers in the organ systems that lead to autoimmune diseases. Background & Aims: We aimed to discern the relative risk for several chronic inflammatory conditions in patients with ulcerative colitis (UC) and Crohn’s disease. Methods: We used the population-based University of Manitoba IBD- Database that includes longitudinal files on all patients from all health system contacts identified by International Classification of Diseases, 9th revision, Clinical Modification codes for visit diagnosis. From the provincial database we extracted a control cohort matching the IBD patients 10: 1 by age, sex, and geography. We considered a potential comorbid disease to be present if the patient had 5 or more health system contacts for that diagnosis. The comorbid disease period prevalence was analyzed separately for patients with UC and Crohn’s disease and a prevalence ratio was been comparing comparing the IBD populations with the matched Results: There were 8072 cases of IBD from 1984 to 2003, including UC (n = 3879) and Crohn’s disease (n Both had a significantly greater likelihood of having arthritis, asthma, bronchitis, psoriasis, and pericarditis than population controls. An increased risk for chronic renal disease and multiple sclerosis was noted in UC but not Crohn’s disease patients. The most common non-intestinal comorbidities were identified in arthritis and asthma. Conclusions: The finding of asthma as the most common comorbidity increased in Crohn. s disease patients compared with the general population is novel. These may be diseases with common causes or complications of one disease that lead to the presentation with another. autoimmune diseases.