Hemianopsia and agnosia, the coexistence of a disorder of perception and a disorder of recognition, is exemplified by two patients chosen for their clinical similarity. These two patients each had a stroke, one of the left and the other of the right posterior cerebral artery, resulting right and left hemianopsia respectively. Primary visual functions such as visual acuity and contrast sensitivity were intact. The differences were only apparent as regards the severity of the agnostic disorders. The patient with the lesion on the left failed to recognize stylized drawings, was alexic, could not lipread and was color anomic, but could easily identify handwriting, faces, and voices. The patient with the right posterior lesion could not recognize handwriting, was prosopagnosic and topographagnosic, but had no difficulty in reading, lipreading, or in recognizing stylized drawings. The apparent double dissociation of function demonstrates the presence of two different visual processing mechanisms which are very probably peculiar to the visual system of the left hemisphere in the one case and to that of the right hemisphere in the other. The authors' findings suggest that the two cerebral hemispheres differ fundamentally in the way they process visual information.