Recent studies showed that SV40 is detected in >40% of non-Hodgkin's lymphoma (NHL) in United States, suggesting SV40-contaminated poliovaccines widely used during the period 1955-1963 to be a major source of SV40 in NHL. We examined the presence of SV40 sequences in 122 cases with NHL and 3 with Hodgkin's lymphoma from Japan. The detection rate of SV40 sequences in diffuse large B-cell lymphoma (19%) was higher than that in peripheral blood cells of normal healthy volunteers in Japan (4.7%; P < 0.05) reported previously as controls for comparison with the study results from cancer patients, suggesting a role for SV40 in the development of diffuse large B-cell lymphoma. In contrast, the frequency of SV40 sequences in NHL cases born between 1951 and 1963 (12%), during which SV40-contaminated poliovaccines might have been inoculated, is not significantly different from that in cases born before 1950 (11%) or after 1964 (15%). SV40 is a new candidate etiologic factor for malignant lymphoma not only in the United States but also in Japan.