Dear Sir,We read with interest the report of De SanJose et al., 1 in which the authors performed a retrospective seroepidemiologic study to determine the presence of polyomavirus SV40 antibodies using an enzyme immunoassay (EIA) in patients with lymphoma in Spain. The authors reported detection rates of 5.9% for SV40 antibodies in patients with lymphoma and 9.5% for controls. Based on these preliminary results, the authors concluded "there was no serological evidence of widespread SV40 infection and no association of SV40 seropositivity with human lymphomas in Spain". It is important to point out that the Institute of Medicine of the National Academies recently concluded "that the biological evidence is strong that SV40 is a transforming virus" and "that the biological evidence is of moderate strength that SV40 exposure could lead to cancer in humans under natural conditions". 2 These conclusions by the Institute of Medicine are in agreement with the recent analyses 3 of controlled molecular, pathologic and clinical data of 1,793 cancer patients that indicated a significant excess risk of SV40 associated with non-Hodgkin's lymphoma [odds ratio (OR) ϭ 5.4, 95% confidence interval (CI) 3.1-9.3] as well as primary brain cancers (OR ϭ 3.8, 95% CI 2.6 -5.7), primary bone cancers (OR ϭ 24.5, 95% CI 6.8 -87.9) and malignant mesothelioma (OR ϭ 15.1, 95% CI 9.2-25.0). Experimental data indicate that SV40 may be etiologically meaningful in the development of a specific subset of human cancers; results include expression of SV40 mRNA and/or T-ag in cancer cells, integration of SV40 sequences in some cancers and complex formation of SV40 T-ag protein with p53 and pRb in some tumor specimens. [2][3][4] These are important findings because SV40 oncogenesis is mediated in large part by T-ag and its effects on tumor-suppressor proteins. [2][3][4] While De SanJose et al. 1 investigated an aspect of the relation between SV40 infection and cancer development in humans, 2 important issues need to be considered. First, studies conducted over the last 3 decades have indicated that SV40 infections occur in children and adult populations today, but the prevalence of infections in any particular population is not known, with available data being limited and inconclusive. 2 The discovery of polyomavirus SV40, as well as its introduction as a pathogen into the human population, was tied to the development and broad distribution of early forms of the polio vaccine. 2,4 Inactivated (Salk) and live attenuated (Sabin) forms of polio vaccine were inadvertently contaminated with SV40 because primary cultures of kidney cells derived from rhesus monkeys, which are often naturally infected with SV40, were used to prepare them. 2,4 Some infectious SV40 survived the vaccine inactivation treatments, and these contaminated vaccines were used in the United States and other countries. 2,4 Contaminated polio vaccines reportedly were used in Spain. 1 The actual number of people infected with SV40 through the use of any contaminated polio vaccine is not kn...