During the period 1968-73, 46 patients with subacute sclerosing panencephalitis (SSPE), a fatal childhood disease related to rubeola, were encountered in Israel. The incidence per million population was 3.4 for Sephardic Jews, 3.2 for Arabs and only 0.5 for Ashkenazic Jews. An environmental factor which sharply demarcated Arabs and Sephardic Jews from Ashkenazic Jews, and which might account for the differences in incidence of SSPE, was family size: 42.3% of Arab families and 21.5% of the families of Sephardic Jews but only 1.1% of Ashkenazic Jewish families had five or more children. It is postulated that older siblings in large families might constitute vectors which introduce rubeola to the younger siblings at a time when they are at unusual risk of developing SSPE (i.e. before two years of age), whereas in small families, rubeola tends to be acquired later in childhood when the risk of SSPE is reduced.