An association between infectious mononucleosis (IM) and MS has been proposed. In a historical prospective study we used records from the Danish State Serum Institute on heterophile antibody (HA) tests for IM performed in all Danish patients over a number of years. Included in the analysis were 6853 HA-positive persons analyzed from 1968 to 1978 (except 1975) and 12,886 HA-negative per sons analyzed in the years 1968, 1969, 1970 and 1978. A search for these persons in the central nationwide Danish Multiple Sclerosis Registry (DMSR) was performed. Among the HA-positive persons 16 cases of MS which met the diagnostic criteria were found with onset of MS after the year of the HA test and before follow-up on 1 January 1991. The expected number for a Danish population, matched by sex, age and year at start of observation, was 5.70 (P < 0.05), the risk ratio being 2.81. No patient had developed MS before contracting IM. Among the HA-negative persons 12 were registered with onset of MS after the year of the HA test and before follow-up, the expected number being 10.47 (P > > 0.05). Although Epstein-Barr virus is not suggested in itself to be the cause of MS, we propose that it is a co-factor in the pathogenesis of this disease.