We report five patients who had multiple sclerosis (MS) associated with type I diabetes mellitus (TODM). Examination of the Israeli National Neurological Disease Register revealed the prevalence of TODM among 334 MS patients under the age of 30 years to be 8.98 per 1,000. This is significantly higher than the prevalence of TODM in the general population of Israel for the same age group, which is 0.095 per 1,000. MS and TODM share clinical, epidemiological and immunological features, and both diseases have a target site that is neuroectodermal in origin. The association between these two putative autoimmune diseases may suggest a similar pathogenetic mechanism.