Background: Patients with Parkinson's disease (PD) have great difficulty in performing two tasks simultaneously, but the neural contribution to this problem has not been identified. In the current study, we investigated the pathophysiology of dual task performance in PD. Methods: We studied 15 patients with PD and 14 healthy controls. Functional MRIs were obtained before and after practicing dual tasks with different complexities. Results: After practice, 12 normal subjects performed all dual tasks correctly. Twelve patients performed the simpler dual tasks correctly. However, only 3 patients could perform the more complex dual task correctly. Dual tasks activated similar brain regions in both groups. The bilateral precuneus was additionally activated during performance of dual tasks compared with the component tasks in both groups. Patients had greater activity in the cerebellum, premotor area, parietal cortex, precuneus and prefrontal cortex compared with normal subjects. Conclusions: Difficulty in performing two tasks simultaneously in patients with PD is probably due to limited attentional resources, defective central executive function and less automaticity in performing the tasks. Practice can diminish dual task interference and improve performance in patients with PD.