We report syntactic comprehension performance of a left handed man with a right-hemisphere infarct. He was unable to accurately map grammatical categories (subject, object) onto thematic roles (agent, patient), despite demonstrating intact conceptual knowledge of these thematic roles. He performed poorly on both active and passive reversible sentences. His asyntactic thematic role assignment cannot be accounted for by a short term memory impairment or any hypothesis that predicts selective vulnerability to passive sentence constructions. Rather than performing randomly, our patient used a temporal or spatial strategy in assigning thematic roles. Because he also had a production-mapping deficit and used the same temporal-spatial strategy in production tasks, we hypothesize that the mapping of thematic roles onto grammatical categories and vice versa may be a specific aspect of sentence processing that is common to sentential production and comprehension. We also raise the possibility that thematic roles have underlying spatial representations prior to being elaborated by grammar. © 1995 Academic Press, Inc.