The Consensus Conference on Brain Tumor Definition was facilitated by the Central Brain Tumor Registry of the United States and held on November 10, 2000, in Chicago, Illinois, to reach multidisciplinary agreement on a standard definition of brain tumors for collecting and comparing data in the U.S. The Brain Tumor Working Group, convened in 1998 to determine the status of brain tumor collection in the U.S., outlined 4 recommendations of which the first 2 guided the discussion for the Consensus Conference: (1) standardization of a definition of primary brain tumors that is based on site alone, rather than on site and behavior, and that can be used by surveillance organizations in collecting these tumors; and (2) development of a reporting scheme that can be used for comparing estimates of primary brain tumors across registries. Consensus was reached on the collection of all primary brain tumor histologies found and reported in the brain or CNS ICD-O site codes (C70.0-C72.9 and C75.1-C75.3), including those coded benign and uncertain as well as those coded malignant. In addition, a comprehensive listing of histologies occurring in the brain and CNS, based on the CBTRUS grouping scheme, was formulated to provide a template for reporting in accordance with the second recommendation of the Brain Tumor Working Group. With consensus achieved on the first 2 recommendations, the stage is set to move forward in estimating additional resources necessary for the collection of these tumors, including funding, training for cancer registrars, identifying quality control measures, and developing computerized edit checks, as outlined in the last 2 recommendations of the Brain Tumor Working Group.