OBJECTIVE: To assess the occurrence of weight gain in patients with Parkinson's disease, with an average 16 months of follow-up after subthalamic nucleus deep brain stimulation. METHODS: We used dual x ray absorptiometry to evaluate changes in body weight and body composition in 22 patients with Parkinson's disease (15 men and seven women) before surgery, 3 months after surgery and on average 16 months after surgery. RESULTS: No patient was underweight before surgery and 50% were overweight. By contrast, 68% were overweight or obese 3 months after surgery and 82% after 16 months (p<0.001). For men, the mean increase in body mass index (BMI) was 1.14 (0.23) kg/m(2) 3 months after surgery and 2.02 (0.36) kg/m(2) 16 months after surgery. For women, the mean increases in BMI at the same evaluation times were 1.04 (0.30) kg/m(2) and 2.11 (0.49) kg/m(2). This weight gain was mainly secondary to an increase in fat mass in both men and women. Three months after surgery, acute subthalamic deep brain stimulation induced an improvement in parkinsonian symptoms (evaluated by the Unified Parkinson Disease Rating Scale (UPDRS) part III) by 60.7 (2.9)% in the "off" dopa condition and a dramatic improvement of motor complications (dyskinesia duration: 82.8 (12.8)%, p<0.0001; off period duration: 92.7 (18.8)%, p<0.0001). CONCLUSION: Although subthalamic nucleus deep brain stimulation significantly improved parkinsonian symptoms and motor complications, many patients became overweight or obese. This finding highlights the necessity to understand the underlying mechanisms and to provide a diet management with a physical training schedule appropriate for patients with Parkinson's disease.