This paper proposes a new cognitive model to explain the aetiology of delusions irrespective of diagnosis and/or phenomenology. The model hypothesises the influence of two processes in the formation and maintenance of delusions; (i) impaired perceptual abilities, particularly affect perception, which fosters the encoding of (ii) idiosyncratic semantic memories, especially those with an affective/self-referential valence. Previous research has established that schizophrenia patients with delusions have impaired semantic memory function. In the current paper we sought to provide evidence for (ii) abnormal semantic processing in persons with delusions with an alternative aetiology. Performance of four cases with a significant delusion post a traumatic brain injury was examined on a broad range of semantic memory tests. Overall semantic processing was impaired in the four cases relative to a normative healthy control sample. Cases performed better on tasks which required categorical identification, relative to the novel production of semantic information, which was poor in all four of the cases. These data offer preliminary evidence for our hypothesis of impaired semantic processing in persons with delusions. Findings will need to be empirically verified in larger sample groups and in those with alternative aetiologies.