We studied two families in each of which three or more individuals were affected by brain tumors. In the first family, which had no evidence of neurofibromatosis or tuberous sclerosis, a man, his sister, and her son developed histologically proven gliomas; the man's great uncle was historically reported to have died from a brain tumor, but the exact nature of the tumor was not known. In this family two of the tumors were low grade astrocytomas of the cerebrum, whereas the third was a mixed glioma of the cerebellum. Karyotypic analysis of this tumor showed no marker chromosomes. A second family had a history of an unusual concentration of brain tumors. In one patient the tumor was a histologically verified glioma. Four other patients had historically reported brain tumors, the descriptions of which suggested gliomas. Both families showed involvement of individuals in adjacent generations, although in both instances there were skipped generations. Twins, siblings, or parents and children are the kindred groups affected in most other reported families with multiple brain tumors. The mode of inheritance of brain tumors in these two families and recent literature on the conditions associated with familial brain tumors are discussed.