Background: Multiple sclerosis (MS) and other autoimmune diseases might cluster. Our aim was to estimate the relative risk (RR) of other autoimmune diseases among MS patients and their first-degree relatives in a population-based cohort study. Methods: Using the Danish Multiple Sclerosis Register, the Danish Hospital Discharge Register, and the Danish Civil Registration System, we estimated RRs for 42 different autoimmune diseases in a population-based cohort of 12 403 MS patients and 20 798 of their first-degree relatives. Ratios of observed to expected numbers of autoimmune diseases, based on national sex-, age-, and period-specific incidence rates, served as measures of the RRs. Results: Compared with the general population, MS patients were at an increased risk of developing ulcerative colitis (RR = 2.0 (95% confidence interval (Cl): 1.4-2.8), n=29) and pemphigoid (RR = 15.4 (Cl: 8.7-27.1), n = 12) but at reduced risk of rheumatoid arthritis (RR = 0.5 (Cl: 0.4-0.8), n = 28) and temporal arteritis (RR = 0.5 (Cl: 0.3-0.97), n = 11). First-degree relatives of MS patients were at increased risks of Crohn's disease (RR=1.4 (Cl: 1.04-1.9), n=44), ulcerative Colitis (RR = 1.3 (Cl: 0.99-1.7), n = 51), Addison's disease (RR = 3.4 (Cl: 1.3-9.0), n = 4), and polyarteritis nodosa (RR = 3.7 (Cl: 1.4-10 0), n = 4). Conclusion: Patients with MS and their first-degree relatives seem to be at an increased risk of acquiring certain other autoimmune diseases.