The relation between the severity of diabetes mellitus (DM) and the risk of significant coronary artery lesions were studied in 7,655 patients undergoing coronary arteriography for suspected coronary artery disease (CAD) between 1972 and 1982. The principal treatment regimen for DM was used to estimate the severity of DM. DM treated with insulin was defined as the most severe (n = 244), followed by DM treated with oral agents (n = 344) and with diet only (n = 380); 6,687 patients did not have DM. Severity of DM in patients with CAD (70% or greater diameter stenosis) was compared with that in control subjects without CAD (0% stenosis) for each of 9 anatomic locations (proximal, middle and distal portions of right, anterior descending and circumflex coronary arteries) using a retrospective case-control approach. The risk of CAD was highest in patients with DM treated with insulin (odds ratio estimate of the relative risk [OR = 3.0]), followed by patients with DM treated with oral agents (OR = 1.8) and lastly in those treated with diet alone (OR = 1.4). Severity of DM was a significant (p less than 0.05) independent predictor of CAD in a multivariate logistic regression model, whereas age at onset and duration of DM were not. The relative risk of CAD was the same (p greater than 0.05) for each of the 9 coronary segments. The data suggest that the risk of CAD increases with the severity of DM, which was a stronger predictor of CAD than duration of DM.(ABSTRACT TRUNCATED AT 250 WORDS)