We have studied the epidemiologic characteristics of insulin-dependent (Type 1) diabetic patients aged 0-19 in a city (San Diego, southern California, USA) characterized by an impressive racial diversity and especially mild and constant climatic conditions. Ascertainment was through retrospective review of medical records in 19 hospitals. For the 3 years 1978-1981 the mean annual incidence of diabetes was 7.3 cases/100,000, with no statistical difference between the sexes. The observed incidence rates in the various ethnic groups was significantly different from expected (p less than 0.03), with an excess of cases among Caucasians and fewer than expected cases among Mexicans, Blacks and Orientals. There was no identifiable seasonal trend. Some of the clinical characteristics at diagnosis differed between the sexes: males were slightly older (9.3 +/- 5.2 years versus 8.8 +/- 3.9 for females), had a shorter duration of diabetes-related symptoms and a higher frequency of infections both at the time of diabetes diagnosis and in preceding months. Females tended to have a higher frequency of Type 1 diabetes in first-degree relatives. This study documents for the first time that, among multiple racial groups living in the same environment, Caucasians are at the highest risk of developing juvenile-onset Type 1 diabetes.