Objective: To test the hypothesis that low serum folate concentrations are associated with an increased risk of acute coronary events in men free of prior coronary heart disease. Setting: Research Institute of Public Health, University of Kuopio, Kuopio, Finland. Design: Prospective study in a cohort of 734 men aged 46 ± 64 y examined in 1991 ± 1993 as part of the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD) and followed for 5 y and 3 months. Intervention: Acute coronary events during the follow-up period were obtained by national hospital discharge registry. Baseline serum folate concentrations were measured by radioimmunoassay. Results: During the follow-up, six (2.5%) men with higher serum folate concentrations (highest third >11.3 nmol=1) and 28 (5.7%) men with lower serum folate (two lowest thirds) developed an acute coronary event (P ˆ 0.008). In a Cox model adjusting for age, examination years, and plasma lycopene concentration, in men with higher serum folate concentrations the relative risk for an acute coronary event was 0.31 (95% CI 0.11 ± 0.90, P ˆ 0.031) when compared with men with lower serum folates. Conclusion: This prospective cohort study in middle-aged men from eastern Finland indicates that moderate-tohigh levels of serum folate are associated with a greatly reduced incidence of acute coronary events. Descriptors: myocardial infarction; acute coronary event, folate; homocysteine; cohort studies; prospective studies; Finland