The authors investigate the postonset hospitalization rate and risk factors during 1987-1994 in Wisconsin, in a population-based, incidence cohort followed from diagnosis of Type 1 diabetes mellitus at ages 0-29 (n = 577). The overall rate was 8.9 +/- 0.60 (standard error) per 100 person-years of diabetes, whereof 5.7 was due to hyperglycemia, 1.9 to hypoglycemia, and 1.3 to other and undetermined causes. Major risk factors for hospitalization were longitudinally measured glycosylated hemoglobin level (rate ratio = 1.5 per 2% increase, 95% confidence interval 1.4-1.7), black/other race (rate ratio = 1.9, 95% confidence interval 1.0-3.6), diagnosis in a non-university-based setting (rate ratio = 1.9, 95% confidence interval 1.2-3.2), female sex (rate ratio = 1.5, 95% confidence interval 1.0-2.4 at age 11), age in males (rate ratio = 0.6, 95% confidence interval 0.4-0.8 per 5-year increase), and public or no insurance up to 18 months postdiagnosis (rate ratio = 2.2, 95% confidence interval 1.1-4.4). For individuals less than 18 years, "black/other race" was replaced in the model by "having other than two biologic parents in the home" (rate ratio = 2.0, 95% confidence interval 1.1-3.5). Hence, hospitalization is common in children, adolescents, and young adults with diabetes, primarily for problems with glycemic control.