Brain tumors were found in 42 mice from a total study population of 77,410 mice, which included several strains (BALB/c, C3H, C57BL/6, and hybrids of these strains). The brain tumors were classified on the basis of the new World Health Organization classification of human brain tumors. Tumors originated from neuroepithelial and meningeal tissues, blood vessels, and germ cells. The youngest animal with a tumor was 111 days of age. The tumor incidence was low, being 0.054% of the total study population, with 0.067% in controls and 0.052% in treated mice. Lipomas were the most common type of tumor diagnosed, and they were considered to represent hamartomas rather than true neoplasms. There were 27 brain tumors other than lipomas, the majority (16) of which occurred in BALB/c mice, whereas meningeal tumors (6) were confined to C3H and hybrid strains of mice. The morphologic characteristics of each tumor type are presented.