The pattern of finger grip formation during natural prehension movements was described in normal subjects with the help of a quantified film technique. Movements were studied in one condition with visual feedback from the moving hand available, and one condition without visual feedback. The studied parameters, including the maximum size of the anticipatory grip and the final size of the grip before contact with the object, were not affected by shifting from one condition of visual feedback to the other. The same technique was applied to a group of patients with cerebral lesions. In two patients with unilateral lesions involving the motor cortex, grip formation with the hand contralateral to the lesion, was found to be severely affected, in that fingers and particularly the index finger, remained stretched until contact with the object was made. In two patients with unilateral lesions in the posterior parietal cortex, grip formation of the contralateral hand was absent specifically in the no-visual feedback condition. The same result was obtained in two other patients with a lesion (subcortical in one case, cortical in the other) of somatosensory pathways corresponding to one hand. These results are interpreted as evidence for the role of cerebral cortex in the control of finger grip formation during prehension of visual objects. Integration at cortical level of visual and somatosensory cues from the involved hand is a necessary condition for grip formation to be adapted to the grasp.