Asymmetrical dimethylarginine (ADMA) is an endogenous nitric oxide synthase inhibitor, which has been suggested to be a novel independent risk factor for endothelial dysfunction and coronary heart disease. We investigated the association of ADMA concentration in serum with risk of acute coronary events. We did a prospective, nested, case-control study in middle-aged men from eastern Finland. In an analysis of men who did not smoke, those who were in the highest quartile for ADMA (>0.62 micromol/L) had a 3.9-fold (95% CI 1.25-12.3, p=0.02) increase in risk of acute coronary events compared with the other quartiles. Our findings suggest that ADMA is a predictor of acute coronary events.