The mortality rate from ischaemic heart disease (I.H.D.) has increased in young women by about 50% in 12 years, and it is now possible to report the findings in 150 women who developed symptoms and signs of I.H.D. under the age of 45. Data obtained from 145 of these women form the basis of this report: 81 presented with myocardial infarction and 64 with angina. In the remaining five there was a definite nonatherosclerotic cause for the premature onset of I.H.D.Hypercholesterolaemia, hypertension, or excessive cigarette smoking each occurred in a large minority, and more than one of these major risk factors was present in most patients. Hypercholesterolaemia was the commonest factor. In women in whom lipoprotein typing was undertaken the type II pattern was more frequent than type IV. The prevalence of hypercholesterolaemia and hypertension was the same in those with myocardial infarction and in those with angina.Excessive cigarette smoking was more common in women with myocardial infarction than in those with angina. The latter did not differ in their cigarette smoking habits from the normal population.A premature menopause had occurred in 20% of these women, but there was no relation between the early onset of I.H.D. with age at menarche, parity, or the incidence of abortion. Oral contraceptives did not increase the risk of myocardial infarction unless one of the major risk factors was also present.Altogether 75% of patients with angina or myocardial infarction survived 12 years. Coexisting hypertension worsened the prognosis. The prognosis after myocardial infarction was similar in these women to that previously described for men under the age of 40.