PIP: During 1968-69, 23,000 women taking oral contraceptives (OCs) and an equal number of controls were recruited by 1400 general practitioners (GPs) throughout the UK. Every 6 months the GPs report on the health and OC use of women in the continuing study. To determine the incidence of arterial disease the number of reports of initial vascular illnesses in each OC-usage group is divided by the calendar months of observation for women in that group. The standardized incidence rate (number of cases) of ischemic heart disease was 0.77 for current OC users, 0.63 for former users, and 0.54 for controls and of cerebrovascular disease for OC users was 0.62 for current users, 0.50 for former users, and 0.20 for controls. The only subcategory for which current users had a significantly increased relative risk was acute myocardial infarction (2.0). Current users also had increased risk of cerebral thrombosis, cerebral embolism, and transient ischemic attacks. Women aged 35 years and over had higher rates of arterial disease than did younger women and cigarette smoking increased the risk for older women in each OC usage group, having little effect on women under 35 years of age. Women over 35 who smoked had a 3.1 times greater risk of developing arterial disease than did nonsmokers. Neither ischemic heart disease nor peripheral vascular disease was shown to be associated with duration of OC use. It was also found that women who smoked and had used OCs had case-fatality rates 2-3 times greater than women in other groups. General conclusions are: 1) the relative risks of OC use are lower for the incidence of 1st events of arterial disease than for deaths, and 2) for cerebrovascular disease there is an increased risk in former users which remains high for over 6 years after stopping OCs.