A patient with downbeat nystagmus was examined by F-fluorodeoxyglucose-positron emission tomography once while off and twice while on successful treatment with 4-aminopyridine. All positron emission tomography scans of the patient showed a reduced cerebral glucose metabolism bilaterally in the region of the cerebellar tonsil and flocculus/paraflocculus when compared with a normal database of the whole brain. An additional region-of-interest analysis revealed that 4-aminopyridine treatment lessened the hypometabolism. This finding supports the hypothesis that the cerebellar tonsil and (para-) flocculus play a crucial role in downbeat nystagmus. The hypometabolism might reflect reduced inhibition or even disinhibition of the circuits to the vestibular nuclei, thus causing downbeat nystagmus. The reduced hypometabolism during treatment probably indicates an improvement of the cerebellar inhibition.