The Epidemiology Shared Resource (ESR) supports the patient data access and consulting needs of Consortium investigators who conduct population-based studies of the etiology, treatment, psychosocial sequelae, and prognosis of cancer. The ESR achieves this aim primarily by providing access to, and scientific expertise about, population-based data on cancer diagnoses collected by the Cancer Surveillance System of western Washington (CSS). The CSS, housed at the Hutchinson since 1974, has been a key component of population research conducted by Consortium investigators, serving as the basis for an increasing number of epidemiologic, molecular genetic, and prevention studies of cancer patients. The ESR provides the infrastructure to coordinate access to the CSS, to develop policies and procedures for use of its confidential patient information, and to implement informatics tools that enhance the ability of investigators to conduct research on )opulation-based series of cancer patients.