To examine the relation between myocardial infarction and cigarette smoking in young women, we investigated the smoking habits of women under the age of 50 who had survived a recent myocardial infarction. They had not been using oral contraceptives, and other identifiable risk factors were excluded. Among 55 such women and 220 control matched for age and area of residence, the proportions of cigarette smokers were 89 per cent and 55 per cent respectively (P less than 0.001). A dose-response relation was evident; among women smoking 35 or more cigarettes per day the rate of myocardial infarction was estimated to be some 20-fold higher than among those who had never smoked. This study demonstrates that cigarette smoking is a risk factor for myocardial infarction in young women who are otherwise apparently healthy.