A 35-year-old woman complained of acute progressive speech and gait disturbance after an upper respiratory tract infection. Neurological examination revealed cerebellar dysarthria, horizontal saccadic pursuit and slow saccadic eye movement, marked truncal ataxia, generalized hyperreflexia and mild sensory impairment of the bilateral lower limbs. She was diagnosed as having acute cerebellomyelitis. We considered her status as acute cerebellar ataxia. The neurological signs and symptoms rapidly improved with intravenous administration of high dose of glucocorticoid. Single photon emission computed tomography (SPECT) using 99mTc-HMPAO was performed on the 9th day and 50th day from the neurological onset. It showed an increased accumulation of 99mTc-HMPAO in the bilateral cerebellar hemispheres and vermis on the 9th day in spite of normal findings of Gd-DTPA enhanced MRI. Less accumulation was seen on the 50th day when her neurological signs almost improved. The accumulation may suggest the inflammatory process of the cerebellum. SPECT can be a useful method for evaluating the severity of cerebellitis.