To evaluate the contribution of hematologic factors and long-term inflammation to the development of myocardial infarction at a young age, we measured hematologic variables, including soluble fibrin and high-sensitivity C-reactive protein, in 90 patients who had myocardial infarction and 138 controls </=45 years of age. Plasma levels of soluble fibrin and C-reactive protein were significantly higher in patients than in controls. On multivariate regression analysis, soluble fibrin was the strongest predictor of myocardial infarction at a young age.