Background: Comorbid inflammatory conditions in multiple sclerosis (MS) patients suggest shared risks with MS.Objective: To estimate if the risk of immune-mediated disease in MS patients and their parents is increased.Methods: Swedish register data were analysed using Cox regression to estimate immune-mediated disease risk among 11284 fathers and 12006 mothers of MS patients, compared with 123158 fathers and 129409 mothers of index subjects without MS. Similar analyses were conducted among 20276 index subjects with MS and 203951 without.Results: Parents of patients with MS did not have a significantly altered immune-mediated disease risk. Patients with MS had a consistently raised risk for several immune-mediated diseases: ulcerative colitis, Crohn's disease, type 1 diabetes, psoriasis, polyarthritis nodosa and pemphigoid. The risk was more pronounced for diseases diagnosed subsequent to MS onset.Conclusion: The increased occurrence of other immune-mediated diseases in MS patients may not be due to shared genetic factors and surveillance bias is likely to be the main or possibly the entire explanation. If not entirely explained by surveillance bias, a modestly raised occurrence of comorbid diseases may be due to shared environmental risks or factors related to MS disease characteristics. © The Author(s) 2012.