Infectious mononucleosis is a clinical manifestation of primary Epstein-Barr virus infection. It is unknown whether genetic factors contribute to risk. To assess heritability, we compared disease concordance in monozygotic to dizygotic twin pairs from the population-based California Twin Program and assessed the risk to initially unaffected co-twins. One member of 611 and both members of 58 twin pairs reported a history of infectious mononucleosis. Pairwise concordance in monozygotic and dizygotic pairs was respectively 121% [standard error (s.e.)=19%] and 61% (s.e.=12%). The relative risk (hazard ratio) of monozygotic compared to dizygotic unaffected co-twins of cases was 19 [95% confidence interval (CI) 11-34, P=003], over the follow-up period. When the analysis was restricted to same-sex twin pairs, that estimate was 25 (95% CI 12-53, P=002). The results are compatible with a heritable contribution to the risk of infectious mononucleosis. © Copyright Cambridge University Press 2011.