BACKGROUND Postural instability is a hallmark of Parkinson's disease. Objective metrics to characterize postural stability are necessary for the development of treatment algorithms to aid in the clinical setting. OBJECTIVE The aim of this project was to validate a mobile device platform and resultant three-dimensional balance metric that characterizes postural stability. METHODS A mobile Application was developed, in which biomechanical data from inertial sensors within a mobile device were processed to characterize movement of center of mass in the medial-lateral, anterior-posterior and trunk rotation directions. Twenty-seven individuals with Parkinson's disease and 27 age-matched controls completed various balance tasks. A postural stability metric quantifying the amplitude (peak-to-peak) of sway acceleration in each movement direction was compared between groups. The peak-to-peak value in each direction for each individual with Parkinson's disease across all trials was expressed as a normalized value of the control data to identify individuals with severe postural instability, termed Cleveland Clinic-Postural Stability Index. RESULTS In all conditions, the balance metric for peak-to-peak was significantly greater in Parkinson's disease compared to controls (p < 0.01 for all tests). CONCLUSIONS The balance metric, in conjunction with mobile device sensors, provides a rapid and systematic metric for quantifying postural stability in Parkinson's disease.