Making informed decisions about breast and prostate cancer screening requires knowledge of its benefits. However, country-specific information on public knowledge of the benefits of screening is lacking. Face-to-face computer-assisted personal interviews were conducted with 10,228 persons selected by a representative quota method in nine European countries (Austria, France, Germany, Italy, the Netherlands, Poland, Russia, Spain, and the United Kingdom) to assess perceptions of cancer-specific mortality reduction associated with mammography and prostate-specific antigen (PSA) screening. Participants were also queried on the extent to which they consulted 14 different sources of health information. Correlation coefficients between frequency of use of particular sources and the accuracy of estimates of screening benefit were calculated. Ninety-two percent of women overestimated the mortality reduction from mammography screening by at least one order of magnitude or reported that they did not know. Eighty-nine percent of men overestimated the benefits of PSA screening by a similar extent or did not know. Women and men aged 50-69 years, and thus targeted by screening programs, were not substantially better informed about the benefits of mammography and PSA screening, respectively, than men and women overall. Frequent consulting of physicians (r = .07, 95% confidence interval [CI] = 0.05 to 0.09) and health pamphlets (r = .06, 95% CI = 0.04 to 0.08) tended to increase rather than reduce overestimation. The vast majority of citizens in nine European countries systematically overestimate the benefits of mammography and PSA screening. In the countries investigated, physicians and other information sources appear to have little impact on improving citizens' perceptions of these benefits.