A prospective survey of all newly diagnosed insulin-dependent diabetes mellitus (IDDM) children and adolescents aged 0-17 years in Israel was conducted for the years 1989 and 1990. All diabetic clinics in Israel treating young diabetics were contacted and they returned written reports to us. Each clinic was also visited regularly by a member of the team who reviewed the individual charts to obtain data on population origin as well as medical and demographic data. A total of 187 patients were identified (164 Jews and 23 Arabs), giving a total incidence rate of 5.46/10(5). Analysis of the incidence rates by population groups showed that Arabs and Jews originating in Asia had the lowest incidence (2.77 and 4.58/10(5) respectively), followed by Jews whose fathers were born in Israel (5.61/10(5)). The highest incidence was registered for Jews originating from Europe and North America (9.34/10(5)). The female-to-male preponderance ratio was higher in the Jews originating in Asia (2.1) than in Jews originating in Europe and North America (1.2). Comparing the present data with a survey performed for the years 1975-80 we found a statistically significant increase in incidence in all population groups. Our findings strongly suggest an influence of genetic factors on the incidence of childhood IDDM.