Objectives: To investigate prefrontal cortex activation during single- and dual-task treadmill exercises at an imposed speed and a speed selected by patients with Parkinson's disease (PD). Methods: Eleven patients with PD (Hoehn and Yahr's stages I–II) and 11 matched healthy controls (HC) performed six 2-minute tasks on a treadmill: two static cognitive tasks, two single-task walking tests and two dual-task walking tests, with each walking test being performed at an imposed and at a self-selected speed. The hemodynamics in the prefrontal cortex (PFC) were measured continuously using functional near-infrared spectroscopy to evaluate cortical activation. The perceived difficulty of the task, cognitive performance and gait parameters were also assessed. Results: For the walking tasks, the HCs activated their PFC during dual-tasks ( P < 0.01) but not when carrying out single-tasks, unlike PD patients who activated their PFC at a similar level for both the single-task and dual-task at an imposed speed ( P < 0.05); no activation occurred at a self-selected speed. Perceived difficulty of the task was significantly greater among PD patients than among HCs ( P < 0.05) and while executing dual-task more than single-tasks ( P < 0.001). Variability in the pace was higher among PD patients than among HCs ( P < 0.01). Conclusion: Imposing a walking speed on a treadmill for PD patients with mild disease severity increases their PFC activity. However, PD patients were unable to further boost PFC activity when a cognitive task was added to the imposed walking speed.