The prevalence of atherosclerotic vascular disease (ASVD) and its risk factors were investigated in 263 insulin-treated diabetic patients, ages 45 to 64 years, who were older than 30 years when their diabetes was diagnosed. The patients were divided into two groups based on the degree of endogenous insulin secretion capacity: Group A: glucagon-stimulated plasma C-peptide less than 0.20 nmol/l and Group B: C-peptide greater than or equal to 0.20 nmol/l. The age-adjusted prevalence of definite myocardial infarction was significantly higher in Group B than in Group A (16.8% vs. 5.2%, p less than 0.01). A similar difference between Groups A and B was found for definite or possible coronary heart disease (54.6% vs. 32.9%, p less than 0.001) and stroke (9.3% vs. 2.0%, p less than 0.05). In multivariate analysis, high glucagon-stimulated plasma C-peptide level (greater than or equal to 0.20 nmol/l) was positively associated with definite or possible coronary heart disease independently of other cardiovascular risk factors. Our results indicate that among insulin-treated patients with a late onset of diabetes, the prevalence of ASVD is markedly higher in those with persistent endogenous insulin secretion (noninsulin-dependent diabetes) than in those with low or no insulin secretion (insulin-dependent diabetes).