The relationship between menstrual and reproductive factors and subsequent risk of coronary heart disease was investigated in a hospital-based case-control study of 202 women with acute myocardial infarction and 374 control subjects admitted for a wide spectrum of acute conditions unrelated to any of the established risk factors for ischemic heart disease. No consistent association was observed with age at menarche or menopausal status, but women with a lifelong irregular menstrual cycle pattern were at significantly elevated risk of myocardial infarction (relative risk = 1.8, 95% confidence interval = 1.1 to 2.9). No clear trend in risk was evident with the number of livebirths, miscarriages, or induced abortions. However, women whose first pregnancy or livebirth occurred before age 20 years showed elevated risks of subsequent myocardial infarction compared with nulliparous ones (relative risks = 2.3; 95% confidence interval = 1.1 to 4.9), and there was a significant trend of increasing risk with earlier first birth. These associations were evident in both younger and middle-age women and were not explained by allowance for several identified potential confounding factors.