Background: There is evidence that, unlike in typical populations, initial lexical activation upon hearing spoken words in aphasic patients is not a direct reflection of the goodness of fit between the presented stimulus and the intended target. Earlier studies have mainly used short monosyllabic target words. Short words are relatively difficult to recognise because they are not highly redundant: changing one phoneme will often result in a (similar-sounding) different word. Aims: The present study aimed to investigate sensitivity of the lexical recognition system in aphasia. The focus was on longer words that contain more redundancy, to investigate whether aphasic adults might be impaired in deactivation of strongly activated lexical candidates. This was done by studying lexical activation upon presentation of spoken polysyllabic pseudowords (such as procodile) to see to what extent mismatching phonemic information leads to deactivation in the face of overwhelming support for one specific lexical candidate. Methods & Procedures: Speeded auditory lexical decision was used to investigate response time and accuracy to pseudowords with a word-initial or word-final phonemic mismatch in 21 aphasic patients and in an age-matched control group. Outcomes & Results: Results of an auditory lexical decision task showed that aphasic participants were less sensitive to phonemic mismatch if there was strong evidence for one particular lexical candidate, compared to the control group. Classifications of patients as Broca's vs Wernicke's or as fluent vs non-fluent did not reveal differences in sensitivity to mismatch between aphasia types. There was no reliable relationship between measures of auditory verbal short-term memory and lexical decision performance. Conclusions: It is argued that the aphasic results can best be viewed as lexical “overactivation” and that a verbal short-term memory account is less appropriate.