OBJECTIVE To determine the efficacy and tolerability of unilateral thalamic deep brain stimulation (DBS) for patients with medically refractory essential tremor (ET) and the tremor associated with Parkinson's disease (PD). BACKGROUND The tremor of ET and PD may produce functional disability despite optimal medical therapy. Several reports have demonstrated efficacy of thalamic DBS in this scenario. METHODS Preoperative and 3-month postoperative tremor ratings were compared in 33 patients (14 ET and 19 PD) with severe tremor. Evaluations included Unified Parkinson's Disease Rating Scale (UPDRS) scores for PD patients and a modified Unified Tremor Rating Scale in ET patients. Open-label and blinded data (unknown activation status) were obtained. RESULTS ET patients demonstrated an 83% reduction (p < 0.0001) in observed contralateral arm tremor. All measures of tremor including writing samples, pouring tests, subjective functional surveys, and disability scores improved significantly. PD patients demonstrated an 82% reduction (p < 0.0001) in contralateral tremor and significant improvement in disability and global impressions. There was, however, no meaningful improvement in other motor aspects of the disease, and the total UPDRS part II (activities of daily living) score did not change. Adverse events, more common in ET patients, were generally mild and were usually eliminated by adjustment of the device parameters. CONCLUSIONS Thalamic DBS is a safe and effective treatment of ET and the tremor of PD. In PD, its use should be limited to patients in whom high-amplitude tremor results directly in significant functional disability.