Elevated levels of pro-inflammatory cytokines such as IL-1ß and IL-6 have been detected in the cerebellum of Rorasg/sg mice in which a neurodegenerative process occurs during the first postnatal month. This suggests the existence of a microglial reaction in the context of an inflammatory process that would be triggered by the massive neuronal loss. To test this hypothesis, we qualitatively and quantitatively studied the microglial cell population using lectin and nucleosidic diphosphatase labeling of the cerebellum of 30 day-old mice. The massive neuronal loss induces a 11.7 fold smaller size of the Rorasg/sg cerebellum compared to the wild-type. We showed that the Rorasg/sg microglia population is exclusively composed of cells displaying the characteristic morphology of activated cells, with enlarged strongly stained cell bodies and few thick processes, in contrast to microglial cells in the wild-type. The density of microglia is 2.7 fold higher in Rorasg/sg than in wild-type mice (22444 ± 5011 cells/mm3 versus 8158 ± 1584 cells/mm3), although the absolute number is 4-fold smaller. These results show that neurodegeneration in the Rorasg/sg cerebellum leads to persistance of microglial activation while in wild-type it disappears around P10.