Ataxia-telangiectasia (A-T) is a heritable disorder of cerebellar ataxia and oculocutaneous telangiectasias caused by mutation of the ATM gene. The most prominent and consistent neuropathologic finding in the disorder is cerebellar cortical degeneration involving significant loss of granule and Purkinje cells. Several past autopsy studies of A-T patients have also noted large-bodied cells located within the molecular layer of the cerebellar cortex and, noting similarities in morphology between these cells and Purkinje cells, hypothesized that the cells were heterotopic Purkinje cells. This study aimed to test this hypothesis using an antibody that labels Purkinje cells, and also to investigate other cell types in the degenerating cerebellar cortex in A-T. Using the anti-calbindin D-28K antibody to label Purkinje cells in cerebellar tissue from five A-T patients and five age- and sex-matched controls, the study found calbindin-positive heterotopic Purkinje cells in the molecular layer occurring at a significantly higher rate in A-T patients than in controls (P = 0.012). Further immunohistochemistry with the anti-Iba-1 and anti-parvalbumin antibodies showed, respectively, an increase in microglial activity (P = 0.14) and stellate-cell density (P = 0.0048) in the cerebellar cortex of A-T patients versus controls. These data add to the as yet unresolved debate over the origin and significance of heterotopic Purkinje cells in A-T.