Hodgkin's disease displays an intriguing variation in incidence with age. For adult disease, there is striking bimodality in incidence with peaks in young adulthood and in older adulthood. On epidemiologic grounds, there appear to be three different diseases: childhood (0-14 years), young adult (15-34 years), and older adult Hodgkin's disease (55+ years). There are marked geographic variations in Hodgkin's disease incidence, but the variations are different for the three age groups. There are also interesting associations between Hodgkin's disease and markers of social class. For the young adult disease, the higher the socioeconomic status of a person, the greater the Hodgkin's disease risk. Additionally, Hodgkin's disease patients appear to have had fewer of the childhood infectious diseases or to have had these diseases at older ages than controls. These factors suggest an analogy between young adult Hodgkin's disease and Epstein-Barr virus, poliomyelitis, and tuberculosis infections. Thus, the descriptive epidemiology of Hodgkin's disease suggests an infectious disease process underlying its etiology in young adulthood and perhaps in childhood. There is a curious relationship between Hodgkin's disease and Epstein-Barr virus infection. Persons with Epstein-Barr virus infection have an increased risk of developing Hodgkin's disease, and the Epstein-Barr virus infection precedes the development of Hodgkin's disease. The virus has never been isolated from or identified in Hodgkin's disease tissue. The mechanisms underlying this association are unknown and may provide important clues to the etiology of Hodgkin's disease and other lymphomas. It is likely that there is no direct person-to-person spread of Hodgkin's disease. This is suggested by several negative studies of linkages between cases, time-space clustering, and aggregation of exposures at schools. Studies have shown that neither physicians nor nurses, groups with greater likelihood of encountering Hodgkin's disease patients than the general public, have an increased risk. There appears to be familial aggregation of Hodgkin's disease. Siblings of young adult cases are at increased risk, whereas siblings of older adult cases have no increase in risk. Among sibling pairs with Hodgkin's disease, there is a marked excess of like-sex pairs. Like-sex siblings have almost twice the risk of discordant-sex siblings, suggesting an interaction between environmental and genetic factors. Thus, it would be of interest to known the risk to spouses of cases who share environments but who do not share genes.(ABSTRACT TRUNCATED AT 400 WORDS)