AIM: Our objective was to examine the association between the fasting serum insulin level and coronary heart disease (CHD) and cardiovascular disease in 40-year-old men and women. METHODS: In 1976, all of the men and women born in 1936 living in four municipalities in the County of Copenhagen were invited for a health examination. The participation rate was 87% (504 men and 548 women). All of the participants were followed through the Danish register of deaths and the register of hospital admissions until 1993. RESULTS: After controlling for relevant risk factors (sex, fasting glucose, total cholesterol, high density lipoprotein cholesterol, systolic blood pressure, smoking, triglycerides, body mass index, physical activity and alcohol consumption) the P value for the association between the insulin level and CHD or cardiovascular disease was found to be 0.002; the above risk factors did not appear to be intermediate variables in a causal chain from insulin level to the development of CHD or cardiovascular disease. The significant association between body mass index and CHD disappeared when the fasting insulin level was controlled for, and insulin seemed to be an intermediate factor between body mass index and CHD. CONCLUSION: The fasting serum insulin level in both men and women is a very good predictor of the development of CHD and cardiovascular disease even after controlling for relevant confounding factors.