Finland has marked regional differences in the occurrence of coronary heart disease (CHD). Although the causes for these differences in CHD mortality and morbidity in the Finnish population are unknown, it offers an excellent opportunity to investigate the effects of non-insulin-dependent diabetes mellitus (NIDDM) on CHD risk in two populations differing significantly with respect to the occurrence of CHD. Therefore, we carried out a 7-year prospective population-based study including a large number of patients with NIDDM (East Finland: 253 men and 257 women; West Finland: 328 men, 221 women) and corresponding non-diabetic subjects (East Finland: 313 men, 336 women; West Finland: 325 men, 399 women). In both study populations the presence of NIDDM increased significantly the risk for CHD events (CHD mortality or all CHD events including CHD mortality or non-fatal myocardial infarction). Diabetic men had 3-4 fold higher and diabetic women 8-11-fold higher risk for CHD than corresponding non-diabetic subjects. Both non-diabetic and diabetic subjects had odds ratios (East vs West) for CHD events of about 2 indicating a similar East-West difference in the CHD risk. Regional difference was quite similar in men and women. These results imply that factors related to NIDDM, independently of conventional risk factors and the occurrence of atherothrombosis in the background population, must play a major role in the pathogenesis of atherosclerotic vascular disease in NIDDM diabetes.