We recorded spontaneous magnetoencephalographic (MEG) activity and somatosensory-evoked fields (SEFs) with a 24-channel planar SQUIDgradiometer in five patients with hemiparkinsonism. The SEFs of the patients were within normal limits. During tremorless periods, the spontaneous activity over the somatomotor cortex had a frequency peak at approximately 10 Hz in all five patients and another at approximately 20 Hz in three. Tremor dampened the 10-Hz activity in all patients; in three the effect was bilateral. Tremor did not increase MEG activity at the tremor frequency. The suppression of the mu rhythm by the parkinsonian tremor resembled that occurring during voluntary movements in healthy subjects.