The value of the apomorphine test as a predictor of the clinical outcome of deep brain stimulation of the subthalamic nucleus (STN) was evaluated in patients with advanced idiopathic Parkinson's disease (IPD) or multiple system atrophy (MSA). Thirteen IPD patients with severe diurnal fluctuations and one MSA patient not responding to dopaminergic drugs were assessed with the Unified Parkinson's Disease Rating Scale (UPDRS) and the timed finger tapping test (FTT), measured preoperatively on and off apomorphine and postoperatively on and off STN stimulation. UPDRS motor items 20-25 were assessed intraoperatively on and off STN stimulation when the clinically effective target was approached. The motor response to immediate intraoperative and long-term STN stimulation was correlated with results of the apomorphine test. The response to immediate intraoperative STN stimulation was accurately predicted by apomorphine challenge in all 13 IPD patients. Clinical outcome following long-term STN stimulation was correlated significantly with preoperative changes due to apomorphine measured with the UPDRS motor scores (r = 0.7125, P < 0.01) and FTT (r = 0.9276, P < 0.001). Moreover, comparison of long-term STN stimulation to preoperative drug treatment displayed a significant reduction in the duration of off-phases and a significant increase in the duration of on-phases. However, in the single patient with MSA no beneficial response was obtained either to apomorphine or to STN stimulation intraoperatively and during the postoperative externalized test period. Our results indicate that the apomorphine test can predict the outcome of immediate and long-term STN stimulation and may help in the selection of candidates for surgery.