The relation of reproductive factors to risk of myocardial infarction in women aged 45-69 years was examined in a case-control interview study carried out in Massachusetts from 1986 to 1990. Each of 858 cases of first myocardial infarction was age-matched with a control from the same precinct of residence. Conditional logistic regression was used to control the matching factors and the major known and suspected risk factors for coronary heart disease. For parous women compared with nulliparous women, the estimated relative risk of myocardial infarction was 1.8 (95% confidence interval (CI) 1.0-3.3). Among parous women, the relative risk estimate for five or more births relative to fewer births was 1.4 (95% CI 1.0-2.0); the estimate for a first birth before age 20 relative to a later age at first birth was 1.7 (95% CI 1.1-2.6). The greatest increase in risk was observed for women who had both an early age at first birth and five or more children. However, confounding by factors related to socioeconomic status may have contributed to the results. Compared with women who had a natural menopause at age 50 or older, women who reached the menopause before age 45 were at increased risk regardless of type of menopause: The estimated relative risks were 2.1 (95% CI 1.3-3.2), 1.7 (95% CI 1.0-2.7), and 1.7 (95% CI 1.0-2.8) for early natural menopause, bilateral oophorectomy, and hysterectomy with retention of one or both ovaries, respectively. These results suggest that early cessation of ovulatory function, whether due to natural causes or to surgery, increases the risk of myocardial infarction. Age at menarche was not related to myocardial infarction risk.