Simian virus 40 (SV40)--a potent oncogenic virus--has been associated previously with some types of human tumours, but not with lymphomas. We examined human tumours for the presence of specific SV40 DNA sequences by PCR and Southern blotting. Viral sequences were present in 29 (43%) of 68 non-Hodgkin lymphomas, and in three (9%) of 31 of Hodgkin's lymphomas. Viral sequences were detected at low frequencies (about 5%) in 235 epithelial tumours of adult and paediatric origin, and were absent in 40 control tissues. Our data suggest that SV40 might be a cofactor in the pathogenesis of non-Hodgkin lymphomas.