There has been debate as to whether essential tremor has a central origin and over the possible role of the inferior olivary nucleus in its genesis. We used positron emission tomography with radioactive water (H2(15)O) to detect abnormal patterns of cerebral activity associated with this condition, at rest, without tremor, and on posture when the tremor was present. At rest, cerebellar blood flow was significantly increased bilaterally in the group with essential tremor (30-40%) but no increased olivary activity was evident. Essential tremor during arm extension was associated with further abnormal increases in bilateral cerebellar and abnormal red nuclear activation. Again, no olivary overactivity was evident. Voluntary wrist oscillation in control subjects caused only ipsilateral cerebellar activation. We conclude that essential tremor is associated with abnormal bilateral overactivity of cerebellar and red nuclear connections but found no evidence of intrinsic overactivity of the inferior olivary nucleus, as evidenced by raised blood flow.