Subacute sclerosing panencephalitis (SSPE), a disease related to measles (rubeola) infection, was more common in Arabs and Sephardi Jews than in Ashkenazi Jews in Israel. There were no familial aggregates and it is unlikely that genetic differences account for this selectivity. Among several non-genetic factors which might explain the selectivity, family size emerged as a possible risk factor. Family size has not previously been suspected as influencing the risk of SSPE. Preponderance of SSPE in rural areas and among the poor would also be compatible with this idea as family size tends to be larger in rural and lower socioeconomic groups. In large families, there may be a greater change that older siblings will transmit measles to very young siblings. In small families, measles may be acquired from peers at about school age when risk of SSPE may be lower.