Various estimates of the proportion of all cancers attributable to infections have been proposed but none have been numerically justified. We have reviewed the evidence for "causality" with respect to infectious agents linked with cancer and estimated the fraction of all cancer cases concerned that are attributable to it. The etiological fraction was applied to the estimated annual incidence of cancer at each specific site in 1990, and the number of attributable cases was obtained. We estimate that 15.6% (1,450,000 cases) of the worldwide incidence of cancer in 1990 can be attributed to infection with either the hepatitis B and C viruses, the human papillomaviruses, EBV, human T-cell lymphotrophic virus I, HIV, the bacterium Helicobacter pylori, schistosomes, or liver flukes. There would be 21% fewer cases of cancer in developing countries (1,000,000 fewer cases per year) and 9% fewer cases in developed countries (375,000 fewer cases per year) if these infectious diseases were prevented. The attributable fraction at the specific sites varies from 89% of cervix cancers attributable to the papillomaviruses to 1% of all leukemias attributable to human T-cell lymphotrophic virus I.