PURPOSE: To describe the distribution of retinal vascular characteristics and their correlates in people with type 1 diabetes. DESIGN: Population-based study with baseline cross-sectional findings. PARTICIPANTS: Nine hundred ninety-six persons who were diagnosed with diabetes before 30 years of age and who were taking insulin in an 11-county area in south-central Wisconsin participated in the baseline examination from 1980 to 1982. In addition, 225 persons without diabetes participated. METHODS: Retinal photographs of 7 standard fields were taken; lightbox grading was performed to determine arteriovenous (A/V) nicking and focal retinal arteriolar narrowing. Computer-assisted grading was performed from a digitized image of field 1 to determine central retinal arteriolar equivalent (CRAE), central retinal venular equivalent (CRVE), and the arteriole-to-venule ratio (AVR). MAIN OUTCOME MEASURES: Frequency and distribution of A/V nicking, CRAE, CRVE, AVR, and focal retinal arteriolar narrowing. RESULTS: In multivariate analyses, retinopathy severity, mean arterial blood pressure, presence of gross proteinuria, glycosylated hemoglobin, and history of cardiovascular disease were associated with CRAE. Retinopathy severity, age, mean arterial blood pressure, duration of diabetes, glycosylated hemoglobin level, and body mass index were associated with CRVE. With the exception of glycosylated hemoglobin, similar factors were associated with AVR. Age (odds ratio [OR] per 10 years, 2.43 and 2.02) and retinopathy severity (OR per level, 1.14 and 1.21) were associated with focal retinal arteriolar narrowing and A/V nicking, respectively. In persons >/=18 years of age, smoking was associated with CRAE, CRVE, and A/V nicking (OR, 2.67), but not with AVR or focal arteriolar narrowing. CONCLUSIONS: This study documented the frequency and distribution of retinal vascular characteristics and their relationships to various factors in persons with type 1 diabetes.