We evaluated lower extremity isokinetic muscle strength to determine affected muscle groups and their dependence on movement velocity, and to establish the relationship between muscle strength and clinical severity, as well as muscle strength and falls, in Parkinson's disease (PD). Twenty-five patients diagnosed with PD and 24 healthy volunteers were enrolled in this study. Lower extremity muscle strength was measured using an isokinetic dynamometer. Each participant's clinical status was examined in accordance with the Unified Parkinson's Disease Rating Scale; fall history was also recorded. We observed a significant decrease in isokinetic muscle strength in the patient group, especially in both hip and knee flexors and extensors. Decreased muscle strength was independent of velocity, and correlated with clinical severity and falls. Movement velocity-independent lower extremity isokinetic muscle weakness has been observed in patients with PD, especially in the knee and hip joints. The evaluation of isokinetic muscle strength may be a useful tool for the assessment of clinical severity and falls in PD.