Objective - To evaluate whether persons with a history of poliomyelitis are at an increased risk of developing multiple sclerosis (MS). Material and methods - All patients diagnosed with acute poliomyelitis in the greater capital area of Copenhagen, Denmark, between 1919 and 1954 were identified and followed with respect to MS. Information on vital status and diagnosis of sclerosis was obtained through linkage with the Danish Civil Registration System and The Danish Multiple Sclerosis Registry, respectively. Follow-up started on the date of the establishment of the Danish Civil Registration System (April 1, 1968) until death, emigration or December 31, 1996, whichever came first. The observed incidence of MS among polio patients was compared with the expected incidence calculated according to national gender, age and period specific rates of MS. Results - During 149,364 years of follow-up, 19 cases of multiple sclerosis were observed among 5652 polio patients compared with 11.0 expected (SIR = 1.73 (1.04-2.74)). The increased risk of MS was most pronounced in polio patients hospitalized during adolescence. Neither gender nor the acute severity of poliomyelitis modified the risk of MS. Conclusion - Our results are based on small numbers of events, however the findings suggest that the polio patients might be at an increased risk of MS.