An eight-year-old female boxer was referred because of weight loss of several months' duration. On physical examination, cachexia was the only reported abnormality. Neurological symptoms were shown only in the last days preceding death and consisted of altered mental status, compulsive behaviour and left rear proprioception deficit. At post-mortem examination, a voluminous, soft, haemorrhagic mass was found invading the floor of the brain. Based on the morphological features of the tumour, marked parenchymal invasion, extensive necrosis and cellular atypia, the mass was classified as an anaplastic ependymoma. This case report shows similarities to the diencephalic syndrome reported in human paediatric medicine in which the main clinical sign is a profound emaciation in spite of normal or slightly diminished caloric intake. Weight loss and cachexia are clinically relevant problems in small animals and these clinical signs should raise a suspicion, among the other differentials, of a brain tumour, even in absence of neurologic signs.