A community-based epidemiological survey of coronary heart disease and its risk factors was carried out over the period 1984-87 on a random sample of adults aged 25-64 years: 13,723 adults living in Delhi and 3375 in adjoining rural areas. ECG examination and analysis of fasting blood samples for lipids were performed on subjects with the disease and asymptomatic adults free of clinical manifestations. The overall prevalence of coronary heart disease among adults based on clinical and ECG criteria was estimated at 96.7 per 1000 and 27.1 per 1000 in the urban and rural populations, respectively. Prevalences of a family history of coronary heart disease, hypertension, obesity and diabetes mellitus were significantly higher in the urban than in the rural population, and smoking was commoner among rural men and women. Mean levels of total serum cholesterol and low density lipoprotein cholesterol were higher among urban subjects; the mean level of triglycerides was higher in rural subjects. The proportions with total cholesterol levels > 190 mg/dl were 44.1% and 23.0% in urban and rural men, respectively, and 50.1% and 23.9% among urban and rural women, respectively. High density lipoprotein cholesterol levels < 35 mg/dl were found in 2.2% of urban men and 8.0% of rural men compared with 1.6% and 3.5% among urban and rural women, respectively. An abnormal ECG pattern (Q wave or ST-T changes) in asymptomatic individuals is also considered to be a risk factor for coronary heart disease. In asymptomatic adults, 1.7% of urban men and 1.2% of urban women showed abnormal Q waves compared with 0.3% of rural men and 0.4% of rural women. A higher proportion of asymptomatic women showed ST-T changes in both populations. Rural men and women had higher total calorie and saturated fat intakes than urban subjects. Differences in dietary cholesterol intake were marginal. Sodium intake was greater in urban adults. Average daily consumption of alcohol by urban men was 12.7 ml ethanol compared with 2.4 ml in rural men.