To determine the pattern of risk factors for acute myocardial infarction associated solely with women a nested case-control study was carried out on cohort data collected during the Royal College of General Practitioners' oral contraception study. Smoking (adjusted relative risk 1.7 for light smokers and 4.3 for heavy smokers), hypertension (2.4), toxaemia of pregnancy (2.8), and diabetes mellitus (6.9) were associated with a significantly increased risk of myocardial infarction. There was no significant trend of risk with social class. Current use of the pill increased the risk only among women who also smoked (relative risk 20.8 for heavy smokers). Previous use of the pill did not influence the risk of myocardial infarction. If heavy smokers also had a history of toxaemia of pregnancy their risk of myocardial infarction was further increased (relative risk 41.0). Other variables associated solely with women, such as parity, hysterectomy, and hormone replacement therapy, had little effect on the risk of having a myocardial infarction. Overall, smoking was the most important independent risk factor and had a strong influence on risks associated with other factors.