Objective: Deep brain stimulation (DBS) of the subthalamic nucleus (STN) has been reported to be effective in alleviating all parkinsonian motor symptoms and motor complications in moderate-to-advanced Parkinson’s disease (PD). We have studied the effects of unilateral STN lesions on moderate-to-advanced PD. Methods: Eleven patients (eight male, three female) with moderate-to-advanced PD were studied. Before surgery, all patients suffered from one or more motor complications (wearing-off and on-off phenomena, dyskinesias). All patients had Hoehn and Yahr stage 3 or 4 in off-drug condition and were evaluated with the Unified Parkinson's Disease Rating Scale (UPDRS). The follow-up time ranged between 12 and 20 months (mean: 14±2.94 months). STN was identified through CT and MRI scans. STN lesions were performed unilaterally using thermocoagulation. Results: Subthalamotomy induced a marked motor improvement contralaterally, ipsilaterally and axially. Mean “on” time total UPDRS and motor scores decreased from 61.7±19.2 to 31.6±15.7 (p=0.001) and from 26.1±11.1 to 15±11.1 (p=0.02), respectively. Total ‘on’ time period increased by 50% in eight patients but did not change in the remaining three. Nine patients had disabling dyskinesia and six of these patients improved significantly (mean 40%, UPDRS part IV). Compared to pre-surgical doses, the levodopa-equivalent dose was reduced in eight patients, increased in one and left unchanged in the remaining two patients during the post-operative phase. Notably, only one out of eleven patients developed hemiballismus. Hemiballistic movements emerged immediately after the ablation and were ameliorated under treatment within two weeks after surgery. Conclusion: We conclude that subthalamotomy is a safe and effective treatment for moderate-to-advanced PD. In cases in which DBS is not feasible, subthalamotomy may be considered as an alternative procedure. (Archives of Neuropsychiatry 2011; 48: 114-8)