A cohort of 9,454 patients with infectious mononucleosis, serologically confirmed, was followed up to determine their risk of developing lymphoproliferative diseases. A total of 1,759 patients was identified in Scotland from 1959 to 1971 and 7,695 in Sweden from 1952 to 1970. The cancer cases were identified by matching the list of patients with infectious mononucleosis against a list of patients with lymphoma, nasopharyngeal and colon carcinomas registered through 1972 in Sweden and through 1973 in Scotland. A four-fold increase in risk was found for Hodgkin's disease based on 7 cases observed against 1.8 expected. Most of the excess cases of Hodgkin's disease occurred within three years of the diagnosis of infectious moonucleosis and among females. These results are similar to those of four other cohort studies and it is concluded that a positive association exists between infectious mononucleosis and Hodgkin's disease. Several explanations of this association were evaluated. Firstly, it may be that a very small proportion of persons who are in the course of developing Hodgkin's disease are temporarily misdiagnosed as having infectious mononucleosis. Secondly, infectious mononucleosis may increase the susceptibility to a factor which causes or promotes the development of Hodgkin's disease. Thirdly, infectious mononucleosis and Hodgkin's disease may share a common aetiology. Arguments in favour of, or against, each of these explanations are presented.