The epidemiologic similarities between Hodgkin's disease in the young and paralytic poliomyelitis suggest that Hodgkin's disease may be a rare consequence of a common infection, with the probability of oncogenesis increasing with age at the time of infection. In a study of 225 cases and 447 controls 15 to 39 years of age, we investigated the association of Hodgkin's disease with factors in childhood that influence age of exposure to infectious agents. Risk among persons with five or more siblings was nearly half that among those with one or none; risk was also reduced among persons of late birth order. Subjects who had lived in multiple-family homes had half the risk of those in single-family housing. Cases had fewer playmates and better-educated mothers than did controls, and cases had twice the rate of infectious mononucleosis. Risk is therefore associated with a set of factors that tend to decrease or delay early exposure to infections, and this association might be explained by a viral origin of the disease, with age at infection as a major modifier of risk.