Studies of multiple primary neoplasms and their relation to the nervous system should consider two important principles: 1) neoplasms of the nervous system have certain unusual features that distinguish them from tumors occurring elsewhere in the body; and 2) there is good evidence that the various histologic types of nervous system tumors should be regarded as separate diseases. The association of nervous system neoplasms and tumors of other sites may occur in patients with phacomatoses or particular genetic syndromes. In addition, certain nervous system neoplasms may be multicentric in origin. Retinoblastoma and osteosarcoma occur together in the same patient more often than expected by chance, as do meningioma and breast cancer. These relationships are important in that they serve to identify the high risk patient, may provide etiologic clues, may point to the presence of genetic syndromes, and may highlight sites in which subsequent tumors are most likely to develop.