Aromatase is the enzyme that catalyzes the biosynthesis of estrogens. It is implicated in neuroprotection. The present study investigated aromatase expression in the cerebellum of dogs infected with canine distemper virus (CDV), a disease characterized by demyelination in the white matter of the cerebellum. The presence of CDV infection was confirmed on the basis of histopathology and immunohistochemical localization of CDV antigen in glial cells of the white matter. The number of aromatase immunoreactive astrocytes were significantly (p < 0.05) higher in CDV-infected dogs compared to control dogs. The results suggest that astrocytes respond to invasion and persistence of CDV by means of increased estrogen production. The results also suggest that the high level of estrogen expression is maintained similarly throughout all stages of the disease since the number of aromatase immunoreactive astrocytes did not vary during the different stages of CDV infection.