For the period of 1 January 1979 through 31 December 1985, an average annual incidence rate (IR) of 12.1/100,000 (95% confidence interval [10.4/100,000, 14.0/100,000]) of insulin-dependent diabetes mellitus (IDDM) was observed in Jefferson County, Alabama, among people less than 20 yr old. Hospital medical-record review was the primary source of case ascertainment. A large proportion of Black children in the county permitted race-specific analyses. The average annual IR among the Black children was less than half that observed in the White children (7.0/100,000 vs. 15.6/100,000, respectively). Nearly equal numbers of White boys and girls were diagnosed during this period; however, there were three times as many Black girls as Black boys diagnosed. Among the 134 Whites, age-specific annual IRs were highest in the 5- to 9- and 10- to 14-yr age groups. Little variation was observed in age-specific rates among the 41 Black subjects. A seasonal trend was evident in both races, with the fewest cases of IDDM diagnosed in the months of April through June (P less than .001). No association between the incidence of diabetes and income level was found among White or Black children. Significant differences in the epidemiology of IDDM between White and Black children suggest an important avenue for studying the etiology of IDDM.