Cognitive functioning was extensively evaluated in a recent callosum-sectioned patient. A number of standardized and specialized experimental memory tests, as well as a sophisticated hypothesis testing task, were administered both before and after operation. No post-operative deficits were obtained. In fact, this patient showed marked improvement on almost every measure utilized. These results demonstrate that the cognitive processing of complex information is not necessarily dependent on the integrity of the corpus callosum, but rather suggest that cognitive functioning is largely an intrahemispheric process.