Motor incoordination, immune deficiencies, and an increased risk of cancer are the characteristic features of the hereditary disease ataxia–telangiectasia (A-T), which is caused by mutations in the ATM gene. Through gene targeting, we have generated a line of Atm mutant mice, Atm^y/y mice. In contrast to other Atm mutant mice, Atm^y/y mice show a lower incidence of thymic lymphoma and survive beyond a few months of age. Atm^y/y mice exhibit deficits in motor learning indicative of cerebellar dysfunction. Even though we found no gross cerebellar degeneration in older Atm^y/y animals, ectopic and abnormally differentiated Purkinje cells were apparent in mutant mice of all ages. These findings establish that some neuropathological abnormalities seen in A-T patients also are present in Atm mutant mice. In addition, we report a previously unrecognized effect of Atm deficiency on development or maintenance of CD4⁺8⁺ thymocytes. We discuss these findings in the context of the hypothesis that abnormal development of Purkinje cells and lymphocytes contributes to the pathogenesis of A-T.