© 2015 IOS Press and the authors. All rights reserved. Background: Planned and initiated actions frequently need to be terminated in favor of another action. It is known that many individuals with Parkinsons disease (PD) have more difficulty self-initiating movement (i.e., endogenously evoked movement) than moving in response to environmental stimuli (i.e., exogenously evoked movement). However, it is not known if individuals with PD display this same endogenous-exogenous asymmetry when needing to terminate, disengage, and reprogram movements. Objective: This study used a novel reaction time (RT) paradigm to test whether patients with mild PD have subclinical deficits of endogenous movement initiation and endogenous movement reprogramming. Methods: Twelve non-demented individuals with PD on medication and 15 demographically similar healthy control (HC) participants completed an experimental paradigm that examined their RTs (key press) following self-selected valid action preparation (endogenous cues) versus valid exogenously presented cues. The paradigm also assessed participants ability to rapidly stop their endogenous or exogenous preparation following an invalid cue and execute an alternative action (key press). Results: Participants with PD produced similar RTs as controls following endogenous and exogenous valid cues, and following invalid exogenous cues. However, following invalid endogenous cues, PD participants were slower than HC participants to stop an endogenous preparation and execute an alternative action. Conclusions: Despite having mild disease and being on dopaminergic medication, these individuals with PD displayed deficits in motor disengagement and reprograming of self-selected actions. Future studies should examine how this phenomenon influences every day actions, as well as possible treatments for this deficit.