Visually directed arm movements have been studied by film recordings in 10 patients with optic ataxia resulting from unilateral lesions of the parietal region, in 3 cases on the right and in 7 on the left. Half of the patients also underwent visuospatial perceptive tests. The results indicate the following. (1) Optic ataxia is a specific visuomotor disorder, independent of visual space misperception. (2) The proximal and the distal components of the movements are equally affected as shown in reaching and hand orientation tasks. (3) The percentages of spatial and orientation errors quantified, respectively, in these two situations show a different distribution across the different hand-field combinations according to the side of the lesion: whereas the right-damaged patients show a deficit essentially related to a field effect, the left-damaged patients show in addition to the latter an impairment related to a hand effect. These findings suggest that the 2 types of visuomotor mechanisms responsible for the proximal and distal components of visually-directed arm movements are controlled by the parietal cortex and that there should exist a hemisphere asymmetry in the functional organization of these mechanisms. (4) Reconstruction of the lesions drawn from CT scans in 8 of the patients shows a salient and constant involvement of the posterior parietal cortex, always including the intraparietal sulcus and either the superior part of the inferior parietal lobule or more often various parts of the superior parietal lobule. The weak co-occurrence of optic ataxia and hemispatial neglect, and their different lesion sites, indicate a double dissociation between these two symptoms.