The usefulness of high-frequency stimulation of the ventral intermediate nucleus (Vim) as the first neurosurgical procedure in disabling tremor was assessed in 26 patients with Parkinson's disease and 6 with essential tremor. 7 of these patients had already undergone thalamotomy contralateral to the stimulated side, and 11 others had bilateral Vim stimulation at the same time. Chronic stimulating electrodes connected to a pulse generator were implanted in the Vim. Tremor amplitude at rest, during posture holding, and during action and intention manoeuvres was assessed by means of accelerometry. Of the 43 thalami stimulated, 27 showed complete relief from tremor and 11 major improvement (88%). The improvement was maintained for up to 29 months (mean follow-up 13 [SD 9] months). Adverse effects were mild and could be eradicated by reduction or cessation of stimulation. This reversibility and adaptability, allowing control of side-effects, make thalamic stimulation preferable to thalamotomy, especially when treatment of both sides of the brain is needed. Lancet 1991; 337: 403-06.