The role of the cognitive neuropsychologist within the diagnostic team is defined. The different steps that a cognitive neuropsychology diagnosis entails are exemplified by describing the complex case of a patient (BR) with deep dyslexia. A rigorous neuropsychological examination disclosed three independent impairments: impaired sublexical orthography-to-phonology conversion mechanism, defective access to phonological lexicon and asymmetrical abstract orthographic representation. These deficits could be mapped onto and accounted for in terms of the classic dual-route cognitive model of reading. The case of BR demonstrates that neuropsychological formulation should not be limited to labelling behavioural disorders and that a thorough analysis of performance profile offers much more than the dichotomous pass–fail on single tests.