Little information is available about insulin-dependent diabetes mellitus (IDDM) when it occurs among US minorities. The incidence of IDDM among African-American and Hispanic children < 18 years of age was determined in the city of Chicago. Hospital records were used as the primary source of cases, and a small amount of additional data was collected from the medical charts. Cases were drawn from records at 37 hospitals in Cook County, IL. African-American and Hispanic patients using insulin, residing in the city of Chicago, and < 18-years-old at onset were registered. Three secondary sources were used and overall ascertainment was estimated at 86%. There were 413 new cases during the 6-year interval 1985 through 1990. The age-standardized incidence of IDDM was 13.2/100,000 (95% confidence interval (C.I.) 11.8-14.8) among African-Americans and 10.8/100,000 (95% C.I. 9.5-12.3) among Hispanics. Hospital use differed between African-Americans and Hispanics, presumably based on geographic, cultural and financial factors. Diabetes among the first degree relatives of children from both ethnic groups was common, and the most frequently listed co-morbid conditions were asthma and obesity. The risk for IDDM in Chicago is among the highest for both African-origin and Hispanic children worldwide. The prevalence of asthma and obesity parallels the high prevalence of these conditions among non-diabetic children in Chicago. The ongoing epidemic of non-insulin-dependent diabetes mellitus (NIDDM) among African-Americans and US Hispanics is likely to be the reason for the large number of minority IDDM patients who have a first-degree relative with diabetes.