Investigators continue to search for reliable markers of prognosis of breast cancer. For many analyses, laboratory techniques permit the use of archival paraffin-embedded tissue collected years previously and readily linked to clinical and follow-up information. Laboratory investigators have often expressed the need for such a tissue resource. We have developed a publicly available resource of archival breast cancer specimens. The pathological material has been collected and reviewed by investigators at four institutions and currently includes breast cancer specimens from more than 9300 cases. Institutional pathologists reviewed slides and blocks using a common protocol and coding scheme. Clinical information and details of follow-up came from data routinely collected by the institutions' cancer registries. Coded data are maintained centrally in a single database. A subset of the data may be searched on the World Wide Web to determine the availability of cases with specified characteristics. The material collected by this Cooperative Breast Cancer Tissue Resource is generally representative of breast cancer diagnosed in community hospital settings in the United States. Seventy-two percent of the living cases have been followed for at least 5 years, and follow-up status is updated regularly. Interested laboratory investigators may apply to the Resource for the use of these tissues. This Resource is proving valuable to laboratory investigators who require large numbers of specimens for validation studies of prognostic markers of breast cancer.