BACKGROUND: Infectious mononucleosis, which is caused by the Epstein-Barr virus, has been associated with an increased risk for Hodgkin's disease. Little is known, however, about how infectious mononucleosis affects long-term risk of Hodgkin's disease, how this risk varies with age at infectious mononucleosis diagnosis, or how the risk for Hodgkin's disease varies in different age groups. In addition, the general cancer profile among patients who have had infectious mononucleosis has been sparsely studied. METHODS: Population-based cohorts of infectious mononucleosis patients in Denmark and Sweden were followed for cancer occurrence. The ratio of observed-to-expected numbers of cancers (standardized incidence ratio [SIR]) served as a measure of the relative risk for cancer. SIRs of Hodgkin's disease in different subsets of patients were compared with the use of Poisson regression analysis. All statistical tests including the trend tests were two-sided. RESULTS: A total of 1381 cancers were observed during 689 619 person-years of follow-up among 38 562 infectious mononucleosis patients (SIR = 1. 03; 95% confidence interval [CI] = 0.98-1.09). Apart from Hodgkin's disease (SIR = 2.55; 95% CI = 1.87-3.40; n = 46), only skin cancers (SIR = 1.27; 95% CI = 1.13-1.43; n = 291) occurred in statistically significant excess. In contrast, the SIR for lung cancer was reduced (SIR = 0.71; 95% CI = 0.58-0.86; n = 102). The SIR for Hodgkin's disease remained elevated for up to two decades after the occurrence of infectious mononucleosis but decreased with time since diagnosis of infectious mononucleosis (P: for trend <.001). The SIR for Hodgkin's disease tended to increase with age at diagnosis of infectious mononucleosis (P: for trend =.05). Following infectious mononucleosis, the SIR for Hodgkin's disease at ages 15-34 years was 3.49 (95% CI = 2.46-4.81; n = 37), which was statistically significantly higher than the SIR for any other age group (P: for difference =.001). CONCLUSION: The increased risk of Hodgkin's disease after the occurrence of infectious mononucleosis appears to be a specific phenomenon.