A two-year-old rough collie bitch with a five-week history of excess salivation, coughing and dysphagia was examined. The dog had bilateral sensory loss over the whole trigeminal field while motor function of the fifth cranial nerve was preserved. No other neurological abnormalities were detected. The dog was observed over 18 months during which the condition did not progress. It was destroyed 18 months after the onset of trigeminal signs following development of a systemic illness, refractory to treatment and unrelated to the neurological signs. Pathological abnormalities were limited to the three major branches of both fifth nerves and the gasserian ganglia. There was partial loss of myelinated nerve fibres in each branch and also in the spinal tract of the fifth nerve in the brain stem. It was considered that the primary abnormality was in the gasserian ganglion and that the fibre loss was secondary to the neuronal lesion in the ganglia. The motor nucleus of the fifth nerve was normal. No cause could be found for this isolated sensory neuropathy of the trigeminal nerve.