It would be of enormous public health importance if diet and physical activity, both modifiable behavioral factors, were causally related to cancer. Nevertheless, the nutritional epidemiology of cancer remains problematic, in part because of persistent concerns that standard questionnaires measure diet and physical activity with too much error. We present a new strategy for addressing this measurement error problem. First, as background, we note that food frequency and physical activity questionnaires require respondents to report "typical" diet or activity over the previous year or longer. Multiple 24-hour recalls (24HR), based on reporting only the previous day's behavior, offer potential cognitive advantages over the questionnaires, and biomarker evidence suggests the 24-hour dietary recall is more accurate than the food frequency questionnaire. The expense involved in administering multiple 24 HRs in large epidemiologic studies, however, has up to now been prohibitive. In that context, we suggest that Internet-based 24 HRs, for both diet and physical activity, represent a practical and cost-effective approach for incorporating multiple recalls in large epidemiologic studies. We discuss (1) recent efforts to develop such Internet-based instruments and their accompanying software support systems; (2) ongoing studies to evaluate the feasibility of using these new instruments in cohort studies; (3) additional investigations to gauge the accuracy of the Internet-based recalls vis-à-vis standard instruments and biomarkers; and (4) new statistical approaches for combining the new instruments with standard assessment tools and biomarkers The incorporation of Internet-based 24 HRs into large epidemiologic studies may help advance our understanding of the nutritional determinants of cancer.