The association of multiple sclerosis (MS) with infections has been studied in two projects. First, the geographical distribution of MS in Finland has been compared with the areal incidence of 7 childhood infections, rheumatic fever and tuberculosis during several periods. No typical and significant correlations could be noticed. Secondly, the infectious history and serum hemagglutination antibodies against measles in a MS group and a group of matched controls in South-Western Finland were studied. The frequency of the different infections was evenly distributed between the groups. However, the MS sera had significantly higher antibody titres.