To assess the role of subregions of the posterior association cortex in movement control, we recorded movement-related potentials (MRPs) in patients who had lesions centered in the temporal-parietal junction (T- PCx; damaged areas 22, caudal 39, 40, and 42; n = 7), superior parietal cortex (ParCx; damaged areas 5, 7, rostral 39, and 40; n = 5), and posterior association cortex lesions involving both the temporal- parietal junction and the superior parietal structures (PosCx; damaged areas 7, 22, 39, 40, 41, and 42; n = 5) and in 14 age-matched normal controls. MRPs were recorded in a self-paced button-press task in which subjects performed a switch closure with the right, left, or both hands (experiment I, experiment II, and experiment III, respectively) under counter-balanced experimental conditions. Data epochs beginning 1400 msec prior to and extending to 600 msec after each motor response were recorded from scalp sites over the precentral, central, and parietal regions. Normal controls and patients with T-PCx lesions generated comparable vertex maximal, symmetrical readiness potentials (onset 1000 msec), contralaterally enhanced NS′ values (onset 500 msec), and MP values (onset 100 msec) preceding voluntary self-paced movements. Extensive lesions involving the posterior association cortex reduced MRP amplitudes. Patients with selective ParCx lesions also had marked reduction of MRPs under all experimental conditions. The MRP findings coupled with clinical and behavioral data on patients with parietal- cortex lesions indicate that the superior parietal regions are part of a neural system necessary for movement preparation.