Cohort studies of human populations have provided the most convincing evidence of links between exposure to specific agents and cancer. The Korean Multi-center Cancer Cohort (KMCC-II) is a multi-center prospective cohort designed to investigate the relationship between exposures to environmental factors, lifestyle factors, host factors and the risk of cancer in Korea. Data on general lifestyle, physical activity, diet, reproductive factors, and agricultural exposures have been collected by direct interview since 1993. Anthropometric measurements and some clinical laboratory findings have also been recorded. This cohort is characterized by the exclusion of cancer cases at the time of recruitment through cancer screening by physicians. The number of cancer-free subjects in the cohort at present totals some 35,692 men and women aged over 35 (121,856 person-years as of December 2000). Of the KMCC-II, 11,045 subjects who have donated their blood and/or urine sample were classified as the KMCC-I. In order to provide an opportunity to incorporate various biomarkers of exposure and the effect of exposure, as well as information on genetic susceptibility, a biological materials bank with blood (plasma, or serum buffy coat, packed erythrocytes) at -70oC and urine supernatants at -20oC has been established for future studies on cancer etiology. The total number of the KMCC-I is 10,694 for the blood bank and 8,907 for the urine bank (total number of observation in person-year = 47,002). Follow-up for cancer incidence has commenced based on an active surveillance system conducted mainly through telephone interview by health personnel or through diagnosis by physicians in each district, and a passive surveillance system through record linkages between the national cancer registry, the national death certificate system, and health insurance medical records databases in Korea. Moreover, this cohort study is expected to play a significant role in assessing the role of genes in the etiology of cancer. These include the determination of the cost-efficiency and reliability of the nested case-control approach, using the cohort, for exploring gene-disease associations and gene-environmental interactions using reduced sample sizes. Along with other cancer cohorts, the KMCC study could eventually provide convincing evidence on new etiologies of cancer in the Asian-Pacific region.