Relating the meaning of a word to the context in which it is encountered is central to comprehension. We investigated the neural basis of this process. Subjects made decisions based on a semantic property of single nouns. The lack of sentence context created ambiguity, as nouns may have several, unrelated semantic identities. Contrasted with unambiguous decisions about each noun's sound structure, the semantic task resulted in activity in the left superior frontal gyrus (SFG), activity that was dependent on choice reaction time. This identified the left SFG as an executive component of a distributed cognitive system that relates a word's meaning to its semantic context to facilitate comprehension.