In Israel, SSPE has been shown to be much more frequent among Sephardic Jews and Arabs than among Ashkenazic Jews. In the present study, we tried to explore environmental factors that may be of etiological importance and explain these differences in prevalence. The study is a case-control one, which includes 95 patients and 2 groups of controls, with 95 people in each. The general population controls were group-matched to the case group by sex, age, and ethnic origin. The family controls consisted of the sibling closest in age to each patient. A statistically significant positive correlation was found between risk of SSPE and early measles infection, large family, overcrowding in the home, older age of the mother, higher birth order, fewer years of schooling of the parents, fewer cultural activities, and rural place of birth. All these factors are interpreted as contributing to a higher risk of early measles infections, which thus may well be the main risk factor for SSPE.