It has been known for some time that patients with Parkinson's disease (PD) have difficulty executing simultaneous motor acts, yet the pathophysiology underlying this impairment remains uncertain. We examined the possibility that these dual-task deficits stem from defective sensorimotor mechanisms producing interference between the motor programs underlying the two tasks. Patients with PD and normal control subjects were tested on their ability to maintain steady-state force with one hand while performing a force-reaction time task with the other hand. Although performance was worse on the dual-task than on the single-task condition for both subject groups, only the patients with PD demonstrated significant interference between the two tasks. The interference across tasks improved in 87% of the patients with PD when tested after treatment with carbidopa/levodopa (Sinemet). These findings support the contention that, in patients with PD, disturbances in the execution of simultaneous motor performance may be due to sensorimotor disinhibition.