- Oral contraceptive users are at increased risk for both arterial and venous thrombosis, some of which can be fatal. Studies are consistent with the existence of a synergism between cigarette smoking and OC use in the pathogenesis of myocardial infarction in young women. To study the relationship between OC use, cigarette smoking and other cardiovascular risk factors among young women. A systematic sample of military personnel, upon discharge from service in the Israel Defense Forces, was asked to complete a research questionnaire. Body weight and height were measured and body mass index computed. Overall, 16,258 questionnaires were collected and analyzed during this 20 year study. There was a gradual, significant increase in OC use until the mid-1980s, from approximately 45% to 60% (P < 0.001), followed by steady rates of 58-64% since then. In contrast, the rates of smoking decreased significantly in the mid-1980s, from approximately 42% to a nadir of 22% in 1991. Since then, the rate of smoking has increased slowly but steadily to reach a level of 35% in 1999. The OC users were more often of western (Ashkenazi) origin and came from families with more education and fewer siblings. They were more often smokers than non-OC users, and started smoking at a younger age. They had significantly lower BMI than non-users. OC use was nearly identical in groups with or without multiple cardiovascular risk factors (smoking, obesity, family history). Smoking and OC use are strongly associated. Other cardiovascular risk factors (smoking, obesity, family history) do not prevent OC users from smoking or smokers to use OC. We suggest that primary care physicians discourage smoking among adolescent females who wish to start using OC. A thorough medical history should be obtained in order to recognize all risk factors for cardiovascular disease and to provide for appropriate contraception counseling.