High-frequency stimulation (HFS) of the subthalamic nucleus (STN) is a well-established therapy for patients with severe Parkinson's disease (PD), but its mechanism of action is unclear. Exaggerated oscillatory synchronization in the beta (13-30 Hz) frequency band has been associated with bradykinesia in patients with PD. Accordingly, we tested the hypothesis that the clinical benefit exerted by STN HFS is accompanied by suppression of local beta activity. To this end, we explored the after effects of STN HFS on the oscillatory local field potential (LFP) activity recorded from the STN immediately after the cessation of HFS in 11 PD patients. Only patients that demonstrated a temporary persistence of clinical benefit after cessation of HFS were analyzed. STN HFS led to a significant reduction in STN LFP beta activity for 12 s after the end of stimulation and a decrease in motor cortical-STN coherence in the beta band over the same time period. The reduction in LFP beta activity correlated with the movement amplitude during a simple motor task, so that a smaller amount of beta activity was associated with better task performance. These features were absent when power in the 5-12 Hz frequency band was considered. Our findings suggest that HFS may act by modulating pathological patterns of synchronized oscillations, specifically by reduction of pathological beta activity in PD.