A case of a 42-year-old man with a glioblastoma multiforme, who postoperatively was found to present many unusual findings, is reported. The patient''s routine neurological examination showed a right hemiplegia worse in the leg with a very marked grasp reflex in the right hand and mild position sense loss on the right side. There were definite signs of intellectual deterioration. Writing with the left hand was incorrect, both spontaneously and to dictation, while writing with the right hand was normal except for some disturbances caused by the grasp reflex. He named objects placed in the left hand (concealed from vision) incorrectly; he could select them afterward with his left hand by touch or pointing; and he could draw the object afterward with his left hand. Even while giving an incorrect verbal description, he could demonstrate correctly the use of the object being held in the left hand. If an object was placed in one hand (concealed from vision), he could not select it from a group or draw it with the other hand. He frequently performed verbal commands incorrectly with his left hand. The authors feel that the simplest explanation of the phenomena is that the patient behaved as if his 2 cerebral hemispheres were disconnected and that the probable cause of this was a lesion of the corpus callosum. Alternative hypotheses and objections are considered and a brief review is given of relevant earlier clinical observations as well as of the recent work of Sperry on callosal deconnection in animals.