We describe the clinical and MRT findings in a 22-year-old male patient with absence of the cerebellum, an extremely rare anomaly. Clinical features included ataxia of limbs, gait and stance, cerebellar oculomotor signs. MRI evaluation showed minute remnants of cerebellar tissue corresponding to the anterior quadrangular lobules. Nine previous cases of total or subtotal cerebellar agenesis have been reported, including Combettes's first report in 1831. We report an additional case and provide MRI of this unusual anomaly detected during life. Diagnosis in this case was made on the basis of MR findings. To our knowledge, this is the fifth report of this disorder being diagnosed in a living patient. (C) 1998 Lippincott Williams & Wilkins.