Chronic traumatic brain injury is rare in man and has not been previously documented in dogs. This report describes a 2-year-old female American Staffordshire bull terrier that was referred with forelimb and hindlimb ataxia, decreased vigilance and disorientation following repeated aggression and physical abuse by its owner. A diffuse cortical lesion was suspected. Cerebrospinal fluid analysis revealed neutrophilic pleocytosis and computed tomography showed marked widening of the cerebral sulci with mild bilateral ventriculomegaly. The dog was humanely destroyed in view of the poor prognosis. Necropsy examination revealed narrowing of the cerebral cortical gyri and consequent widening of the sulci without distortion or displacement of the neural parenchyma. These features were consistent with bilateral diffuse cortical atrophy. Microscopically, there were chronic subarachnoid haemorrhages and the cortical subpial layer displayed spongiosis, capillary hyperplasia, astrocytosis, microgliosis and frequent neuronal necrosis occurring in a characteristic laminar pattern. This histopathological pattern of damage was significantly different from that previously described in people suffering from repeated traumatic brain injuries over a long period of time.