The trend of increasing testicular cancer incidence is unlikely to be explained by known risk factors such as cryptorchidism. An infectious etiology of testicular cancer has been proposed, based on similarities between the age-specific incidence pattern of testicular cancer and that of other virus-associated malignancies. Only a small number of epidemiologic studies have evaluated this hypothesis, but collective results suggest that exposure to EBV and CMV may relate to testicular cancer risk. Further indirect evidence comes from studies showing increased risk among immunocompromised men, as well as from studies indicating a relation between testicular cancer and history of sexually transmitted diseases. Direct epidemiologic evidence regarding the role of sexually transmitted infectious agents in testicular cancer etiology is limited although suggestive. Using serum collected prospectively, we propose to evaluate the association between antibody response to HSV-2, HPV-16 and 15, HHV-8 and Chlamydia trachomatis on the one hand and the occurrence of testicular cancer on the other. As a secondary aim, we will examine whether the associations between infection with any of these agents and testicular cancer are modified by age. The proposed study is a case-control study nested in a well-defined cohort of men who donated blood samples as part of the JANUS project in Norway. The JANUS project, sponsored by the Norwegian Cancer Society, was established in 1973 for the purpose of prospectively collecting serum to be used in epidemiologic studies of cancer development and cardiovascular disease. As of October 1993, the JANUS serum bank contained 424,938 serum samples from 293,692 individuals, none of whom had cancer at the time of blood sampling. All blood donors to the JANUS serum bank are annually linked, through a unique, individual identification number, with the Cancer Registry of Norway. Eligible cases for the present analysis are male blood donors listed in the Cancer Registry through 1993 with a diagnosis of invasive testicular cancer. For each case, three blood donors, individually matched by age and year at sample collection, have been randomly selected from the JANUS serum bank as controls. A total of 88 case patients and 262 matched controls have been identified and will be included in the proposed analysis. Serum antibody titers against HSV-2, HPV, HHV-8 and C. trachomatis will be determined using enzyme-linked immunosorbent and immunofluorescence assays. Individual antibody titer values will be distributed into quartiles based on the distribution of the entire study population. The data will be modeled through conditional logistic regression, taking into account the individual matching. The JANUS cohort provides a unique opportunity to study infectious agents in relation to testicular cancer risk. To our knowledge, this is the first study to evaluate the viral origin of testicular cancer using serum collected prior to diagnosis.