Aim III. To prospectively determine the reliability, validity and responsiveness of the iTUG mobility score. We will perform a prospective study with 60 patients with PD that compares standard clinical measures of postural instability and gait difficulty mobility (PIGD items from the Unified Parkinson's Disease Rating Scale (UPDRS) and TUG stop-watch scores) with our new iTUG mobility score in the OFF and ON levodopa states. We hypothesize that the iTUG will have good test-retest reliability, be significantly correlated with standard clinical measures and fall history and be more responsive to dopaminergic state than current mobility tests used in clinical trials. This project will develop, produce, and test a novel clinical balance and gait assessment tool, the instrumented Timed Up and Go test (iTUG) using our new, wearable, inertial sensors and our new clinical trial data management server that automatically transmits, stores, and analyzes data collected with the iTUG. By automatically providing a more sensitive, accurate, and comprehensive method to test balance and gait, clinical trials to improve mobility in patients with Parkinson's disease and other neurological disorders will be significantly more effective and efficient.