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Background An inherited predisposition is an important factor in the etiology of myocardial infarction (MI) at a young age. However, the extent of the risk for early-onset MI in relatives of young patients is still unclear, due to the paucity of family history data. Hence familial aggregation of early-onset MI was investigated in a cohort of relatives of Italian patients who had survived MI who occurred at the age of 45 or earlier. Methods In the framework of a case–control study, lifetime data and early-onset MI status for 11,696 relatives of cases and 8897 relatives of controls were collected using a standardized questionnaire. Results Occurrence of early-onset MI in females was very uncommon (Kaplan–Meier risk = 0.6%, 95% confidence interval (CI): 0.38–0.82%, for female case relatives), and significantly lower than that for male case relatives (5.0%, 95% CI: 4.41–5.56%). The hazard ratio (HR) for case relatives was approximately 3-fold greater than that for control aunts (taken as reference category). Risk for early-onset MI to siblings (HR = 1.7, 95% CI: 1.33–2.18) was significantly different from that to parents (HR = 0.9, 95% CI: 0.71–1.16). The familial risk ratio λ R was 2.6 (95% CI: 2.30–2.89) for case relatives, using control parents as reference population for early-onset MI risk estimates (i.e. 37 per 100,000 in fathers and 7 per 100,000 in mothers). Conclusion We evaluated the risk of early-onset MI by category of relatives, obtaining evidence for familial aggregation of the disease in this Italian sample and providing figures for genetic counselling and planning genetic epidemiological studies.