BACKGROUND: The female preponderance in incidence of multiple sclerosis (MS) calls for investigations into sex differences in comorbidity with other autoimmune diseases (ADs). OBJECTIVES: To determine whether male and female patients with MS have a higher frequency of autoimmune comorbidity than controls, and to describe the type and frequency of ADs that are associated with MS. METHODS: Our database was established by linkage of the Danish MS Registry to The Danish National Patient Register and consisted of 1403 patients of both sexes with clinical onset of MS between 2000 and 2004, and 25 matched controls for every case. RESULTS: None of the ADs occurred more frequently in female cases than in controls. Male cases were more likely to have Type I diabetes mellitus (odds ratio (OR) = 3.34; 95% CI 1.40 - 7.02; p < 0.008), Crohn's disease (OR = 5.03; 95% CI 1.18 - 16.10; p = 0.03) and systemic lupus erythematosus (OR = 12.55; 95% CI 1.62 - 69.95; p = 0.02) than male controls. CONCLUSIONS: Autoimmune disorders are rare, but some of them tend to occur together with MS at a higher rate than in controls. Although women are generally more prone to ADs than men, significantly increased occurrence of other ADs were only found in male MS patients.