Epidemiologic data relating multivitamin supplement use to the risk of cardiovascular disease are sparse and inconsistent. We examined the association between self-selected use of low dose multivitamin supplements and the risk of myocardial infarction (MI). Our results are based on data from a large population-based, case-control study of subjects aged 45-70 y residing in Sweden, a country in which consumption of fruits and vegetables is relatively low and foods are not fortified with folic acid. The study included 1296 cases (910 men, 386 women) with a first nonfatal MI and 1685 controls (1143 men, 542 women) frequency-matched to the cases by sex, age and hospital catchment area. Odds ratios (OR) and 95% CI were calculated from unconditional logistic regression models. Among controls, 57% of the women and 35% of the men used dietary supplements; corresponding figures for the cases were 42 and 27%, respectively. Of those taking supplements, 80% used multivitamin preparations. After adjustment for major cardiovascular risk factors, the OR of MI comparing regular users of supplements with nonusers were 0.79 (95% CI 0.63-0.98) for men and 0.66 (95% CI 0.48-0.91) for women. This inverse association was not modified by such healthy lifestyle habits as consumption of fruits and vegetables, intake of dietary fiber, smoking habits and level of physical activity, although never smoking appeared to outweigh the association in women. Findings from this study indicate that use of low dose multivitamin supplements may aid in the primary prevention of MI.