BACKGROUND: Hematocrit is an independent predictor of cardiovascular risk in middle and old age, but whether hematocrit is also a predictor at younger ages is presently not known. In this study, we examined whether hematocrit measured in adolescence was associated with the risk of myocardial infarction later in life. METHODS: During Swedish national conscription tests conducted between 1969 and 1978, the hematocrit was measured in 417,099 young Swedish men. The cohort was followed for subsequent myocardial infarction events through December 2010. Associations between hematocrit and myocardial infarction were accessed using Cox regression models. RESULTS: During a median follow-up period of 36years, 9322 first-time myocardial infarctions occurred within the study cohort. After adjusting for relevant confounders and potential risk factors for myocardial infarction, men with a hematocrit≥49% had a 1.4-fold increased risk of myocardial infarction compared with men with a hematocrit≤44%. This relationship was dose dependent (p<0.001 for trend) and remained consistent throughout the follow-up period. CONCLUSIONS: In this cohort of young Swedish men, hematocrit was associated with the risk of myocardial infarction later in life after controlling for other coronary risk factors. The study findings indicate that hematocrit may aid future risk assessments in young individuals.