Freezing of gait (FOG) is common in advanced Parkinson's disease (PD), is resistant to treatment and negatively impacts quality of life. In this study an ambulatory FOG monitor was validated in 11 PD patients. The vertical linear acceleration of the left shank was acquired using an ankle-mounted sensor array that transmitted data wirelessly to a pocket PC at a rate of 100 Hz. Power analysis showed high-frequency components of leg movement during FOG in the 3-8 Hz band that were not apparent during volitional standing, and power in this 'freeze' band was higher (p=0.00003) during FOG preceded by walking (turning or obstacles) than FOG preceded by rest (gait initiation). A freeze index (FI) was defined as the power in the 'freeze' band divided by the power in the 'locomotor' band (0.5-3 Hz) and a threshold chosen such that FI values above this limit were designated as FOG. A global threshold detected 78% of FOG events and 20% of stand events were incorrectly labeled as FOG. Individual calibration of the freeze threshold improved accuracy and sensitivity of the device to 89% for detection of FOG with 10% false positives. Ambulatory monitoring may significantly improve clinical management of FOG.