The Multiethnic Cohort Study was designed to study prospectively the relationship of diet and other lifestyle factors to the risk of cancer. The cohort was established in 1993-1996 in Hawaii and California (primarily Los Angeles) and included a representative sample of more than 215,000 men and women primarily from five different ethnic groups: Japanese, whites, Native Hawaiians, Latinos, and African- Americans. Because of the emphasis on diet, great attention was paid to developing and pre-testing a self-administered quantitative food frequency questionnaire that would adequately assess food and nutrient intakes in these groups. An extensive food composition database was also created for the study. In addition, during data collection, a calibration study was conducted that makes possible adjustment for measurement error in nutrient intakes and valid comparison of intakes across the several ethnic groups. At the present time, blood and urine specimens are being collected from cohort participants and should yield a biorepository of more than 80,000 subjects. Baseline data indicate that the cohort is well representative of the general population of Hawaii and California, so that results can be generalized. These data also show a wide range in dietary intakes and in other lifestyle variables that should facilitate the testing of etiologic hypotheses.