The organization and structure of the lexicon in a linguistic performance model is here examined, using data elicited from dyslexic patients suffering from focal brain lesions. It is argued that phonological, semantic, and orthographic representations, in separate sub-lexicons, are needed to account for the selective reading and writing disorders. The role of such data and their relevance to linguistic theory are discussed. Linguistic concepts provide the framework for investigating both normal and abnormal (e.g. aphasic) speech production and comprehension. In turn, data from aphasia may provide new insights into the nature of the mental grammar.