Introduction: In this study the effect of subthalamic nucleus neep brain Stimulation (STN-DBS) on the gait parameters of patients with Parkinson's disease (PD) was examined using an ambulatory gait analysis system based on miniature gyroscopes. The aim was twofold: to characterize gait with and without stimulation (ON and OFF states) and to understand how STN-DBS improves motor performance in PD patients.Methods: Ten PD patients, 20±3 months after implantation of bilateral STN-DBS were asked to walk 20m. Patients did the test twice: once STN-DBS was ON and once 180 minutes after turning it OFF. Six miniature gyroscopes were attached on each wrist, thigh and shank respectively. A light data-logger (Physilog®) carried by the patients recorded angular velocities measured by gyroscopes, during each walking trial. Spatio-temporal gait parameters were estimated using a method described earlier [1]. In addition, the range of the movements of lower limbs in the sagittal plane and hands in 3D were estimated based on the integration of angular velocity during each gait cycle. Mean and stride-stride variability of all parameters were obtained for each patients. The same protocol was used to measure ten normal, age matched control subjects. Unified Parkinson's Disease Score (UPDRS III) was obtained for each patient in the ON and OFF states.Results: Compared to controls, PD patients (OFF state) had significantly shorter normalized stride length and velocity (SL=0.40 vs. 0.79, p<0.004 and SP=0.34 vs. 0.80, p<0.002), longer gait cycle time (GC=1.39s vs. 1.01s, p<0.03) and double-support phase (DS=33% vs. 19%, p<0.001). Range of rotations of shanks (SR=41° vs. 75°, p<0.001), thighs (TR=26° vs. 46°, p<0.001), knees (KR=36° vs. 52°, p<0.01) and hands (HR=11° vs. 48°, p<0.001 were significantly shorter in PD patients. Also peak angular velocity of shank was significantly reduced as compared to controls (SV=203°/s vs. 383°/s, p<0.001). STN-DBS significantly improved most gait parameters. Namely SL, SP, SR, TR, KR, HR, SV increased (p<0.005) and DS decreased (p<0.05) but not GC. Yet even with stimulation SL, SP, SR, TR, KR, HR and SV were significantly different from controls (p<0.05). STN-DBS significantly decreased stride-stride variability of SV (p<0.002) but not for GC. Also HR and SR showed significant correlation (r=-0.76 and r=-0.85 with p<0.001) with UPDRS (sub scores 22.b+23+24+25+31 for HR and 29+22.c for SR).Discussion and Conclusions: STN-DBS significantly improved gait parameters but not up to normal values. Stride-stride variability of parameters was significantly different between PD patients in OFF state and control subjects, and STN-DBS had a limited effect on it. Also a good correlation between range of rotation of the limbs and UPDRS was found. Physilog® system offered a simple way to characterize gait in PD patients. It allowed evaluating spatio-temporal gait parameters as well as the range of limbs' movement and provided an efficient way to estimate stride-stride variability.