In 1986, 110,660 of 281,589 residents aged 25-74 years in Da Qing, Hei Long Jiang Province of China, were surveyed. Based on the results of a 75-g oral glucose tolerance test, 630 subjects were found to have previously undiagnosed diabetes according to 1985 WHO criteria. Among them, 600 diabetics aged 35-74 years (288 men, 312 women) and 410 non-diabetics of similar age with normal glucose tolerance (207 men, 203 women) were examined to determine the prevalence of retinopathy and coronary heart disease (CHD) and to evaluate associated characteristics. Retinal examinations of 423 newly diagnosed diabetics showed that 15.4% had several microaneurysms and/or small intraretinal haemorrhage, 5.5% soft exudates, 7.1% hard exudates, and 2.3% proliferative retinopathy. Among 220 non-diabetics, 13.6% had one or two microaneurysms and/or small intraretinal haemorrhage, and only 1.4% had a few soft exudates; half of the non-diabetics with retinopathy had hypertension. CHD, according to Minnesota coding (1.1-1.3, 5.1-5.3 and 7.1) of resting electrocardiograms, was ten times more frequent in the diabetics (3.59%) than in the controls (0.32%), after adjusting for age and sex. Multiple regression analysis showed that plasma glucose concentration was a risk factor for retinopathy after adjusting for age, sex, body mass index (BMI), smoking and blood pressure. Two-hour plasma glucose concentration (after adjusting for age, sex, BMI, smoking and blood pressure) and blood pressure (after adjusting for age, sex BMI, smoking and 1-h or 2-h plasma glucose level) were associated with CHD among the diabetics and non-diabetics and among the diabetics alone. Thus, both micro- and macrovascular complications occur frequently in previously undiagnosed Chinese diabetics and the frequency of CHD is markedly increased compared to the low frequency among Chinese non-diabetics.