We examine two different forms of comprehension impairment, `semantic dementia' and `asyntactic comprehension', focusing on the assignment of thematic roles: the determination of who did it to whom. We show, first, that the loss of word meaning does not impede thematic assignment in semantic dementia, demonstrating that syntactic information, along with some knowledge of the verb, is sufficient for the assignment of thematic roles. Studies of normal subjects indicate, however, that this process is normally subject to semantic influences; asked to judge the plausibility of sentences, subjects respond faster when thematic assignment is semantically constrained. The sentence plausibility judgments of `asyntactic' comprehenders (aphasics with diminished syntactic control over thematic assignment) show increased effects of these semantic constraints. We discuss these results in relation to current issues in sentence processing.