Recent results with normal subjects have shown that the locus of the phonetic boundary of a speech continuum may change as a function of the word/nonword status of the endpoint stimuli. This so-called lexical effect in phonetic categorization has been used as evidence for the role of top-down processing in speech perception. This study investigated whether aphasic patients show a similar influence of lexical status on phonetic categorization. Two test continua were created varying in voice-onset time: in one continuum, the two endpoint stimuli were word/nonword, i.e., "duke"--"tuke," and in the other continuum, they were nonword/word, i.e., "doot"--"toot." Twelve aphasic patients were tested including 6 Broca's aphasics and 6 Wernicke/Conduction aphasics. The subject's task was to determine whether the first sound of the stimulus was a "d" or "t." Broca's aphasics showed a large lexical effect, with the magnitude of the effect being greater than that for normals. These results suggest that the Broca's aphasics place a heavier reliance on a heuristic strategy than on the perceptual information embedded in the test stimuli in making a phonetic categorization. In contrast, Wernicke/Conduction aphasics did not show a lexical effect, suggesting that these patients are less likely than either normals or Broca's aphasics to use heuristic strategies in lexical processing. The overall results are considered in relation to current views on language-processing deficits in aphasia.