The aim of this study was to find out whether there is a seasonal pattern in the month of birth of children with IDDM in Israel and whether this pattern, if present, differs from that of total live births. One thousand and ninety-five out of 1,188 children and adolescents (0-17 years) who developed IDDM in Israel between 1980-1993 and whose month of birth was known were included in the study. Separate analysis was made for Jews (n = 987) who have a high incidence (10-18/105) and Arabs (n = 108) with a low incidence (2.9/105) of IDDM. The pattern of total live birth distribution in Israel over a 20-year period served as control. A significantly different seasonal pattern was found in the two IDDM populations. Whereas the Jews had the lowest number of births in winter (January-March) and the highest in spring (April-June), the Arabs presented an inverse pattern in the first months of the year. The distribution of births of the children who developed IDDM was different from that observed in total live births in the Jewish population. The findings in the Jewish population in Israel (a high incidence group) support the hypothesis that IDDM is triggered in some children by viral infections transmitted by the mother during pregnancy or in the early postnatal period. In the Arab population (a low incidence group) a protective (immune and/or genetic) mechanism may exist.