Sixty aphasic patients and 55 normal control subjects were tested on a sentence production protocol that required subjects to produce specific sentence types from semantic representations. Normal subjects produced the expected targets with great reliability. Analysis of the patients' performance indicated that patients had difficulty producing both grammatical forms and thematic roles. Patients had more trouble producing grammatical elements than content words, and showed differential difficulty on sentence types that had more grammatical elements and in which the order of thematic roles was non-canonical. The results provide evidence regarding the processing load imposed by different components of the sentence production process.