To examine a possible relationship between infectious diseases and multiple sclerosis (MS) an enquiry was carried out among 606 MS patients in Switzerland. The data concerning their infectious childhood diseases were compared with epidemiological data for the normal Swiss population obtained from the Swiss Federal Health Office and from the Institute of Medical Statistics. The mean age of the MS patients was 50.7 years and the mean age at onset of multiple sclerosis was 33.8 years, significantly earlier in women (33.2 years) than in men (35.4 years, p < 0.05). In 18.8% multiple members of the family were affected. In comparison with persons of the control population, MS patients had measles infection at a later age (6.4 vs. 7.5 years). The curve of the age at which several infectious childhood diseases occurred was shifted to higher ages for MS patients (p < 0.005) compared to normal controls for mumps (80.2% for MS vs. 64.1% for controls in the age group 5-14 years), rubella (64.3% for MS vs. 48.4% for controls in the age group 5-14 years) and varicella (81.9% for MS vs. 39.0% for controls in the age group 5-19 years). For pertussis, however, there were more cases among those who later developed MS in the age group 1-9 years, which was earlier than in controls (86.0 vs. 56.7%). These results are compatible with the hypothesis that the risk of developing multiple sclerosis may be associated with acquiring certain infectious childhood diseases at a later stage in comparison to normal controls.