OBJECTIVE: The purpose of this study was to simultaneously evaluate multiple components of disequilibrium in patients with idiopathic Parkinson's disease (PD) in ON and OFF states and healthy age- and sex-matched controls on tests of balance, gait, and dynamometry. DESIGN: Thirty subjects with Parkinson's disease and 30 controls were matched for age and sex. Isokinetic and balance laboratories of a clinical research center were used for assessment. Performance results for static and dynamic balance by dynamic posturography for sensory organization tests (SOT), limits of stability, clinical gait assessment, and dynamometric assessment for the trunk, hip, and ankle at different speeds for concentric muscle strength were obtained. Tests were done both in ON and OFF state in Parkinson's disease patients and results compared. RESULTS: Between OFF state and controls, a significant difference was observed for SOT-2 (proprioception, P < 0.005), SOT-6 (conflicting vision, P < 0.001), and SOT-4 (eyes open with sway support, P < 0.038), and there was less use of ankle strategy in SOT-3 (sway vision, P < 0.04). No significant difference was observed for vestibular function (SOT-5). Significant difference was also observed (P < 0.001) for all variables in limits of stability except for reaction time and for muscle strength of trunk, hip, and ankle (P < 0.001) between OFF state and controls. After antiparkinsonian medications, significant improvement was observed for gait velocity (P < 0.002), muscle strength (P < 0.001), and strategy score in SOT-3 between OFF and ON states. A positive correlation was observed between muscle strength (ankle, hip, and trunk) and gait velocity (ON state r = 0.37, OFF state r = 0.56) and movement velocity (ON state r = 0.39). A positive correlation was also seen between ankle strength and gait velocity in both ON (r = 0.393) and OFF states (r = 0.397) and between ankle strength and ankle strategy in all SOTs except SOT-3 in the OFF state. CONCLUSIONS: The quantitative reduction of muscle strength in the spine, hip, and ankle, along with impaired proprioception, visual sense, and smaller base of support, were the main causes for postural instability in Parkinson's disease patients. A correlation was seen between muscle strength, static and dynamic balance, and gait in both ON and OFF states. In contrast to the previous studies, the present study showed that medications improved the muscle strength, gait speed, and use of ankle strategy but did not worsen proprioceptive sense.