PURPOSE AND DESIGN: This report presents a comprehensive and statistical analysis of the brain tumor-polyposis (BTP) cases referred to as Turcot's syndrome in the literature. RESULTS: BTP patients encompass a heterogeneous group that can be classified into two statistically distinct clinical entities based on phenotype of the polyps (P = .0001), presence of colorectal cancer (P = .0001), type of brain neoplasm, ie, glioma or medulloblastoma (P = .0001), presence of skin lesions (P = .0004) and cafe-au-lait spots (P = .0008), as well as consanguinity (P = .0135). CONCLUSION: The first entity (BTP syndrome type 1) consists of patients who have glioma and colorectal adenomas without polyposis (non-FAP cases), and their siblings with glioma and/or colorectal adenomas. For these patients, we show that the patient's age at malignant glioma occurrence is less than 20 years (50 to 80 years in the general population), which strongly supports the existence of an underlying genetic cause. The neoplasms of these patients show DNA replication errors, which suggests a relationship with hereditary nonpolyposis colorectal cancer (HNPCC), a disease characterized by germline alterations in DNA mismatch repair genes. The second entity (BTP syndrome type 2) consists of patients with a CNS tumor that occurs in a familial adenomatous polyposis kindred (FAP cases). These patients carry germline mutations in the APC gene, which suggests that mutations in this gene might predispose to brain tumors. Risk analysis shows increased incidence of medulloblastoma in FAP patients, but APC mutations are not found in sporadic glioma or medulloblastoma. Therefore, further investigations should establish whether the occurrence of medulloblastoma in an FAP family represents a variant of FAP.