This qualitative study investigated written discourse in the form of personal letters written by ten people with aphasia following stroke and ten people with cognitive-language disorder as a consequence of traumatic brain injury, and compared their performance with 15 non brain-damaged writers. Personal letters perform the dual function of providing information and maintaining social relationships. Using the Systemic Functional Linguistics framework for investigation, letters were examined in terms of their dual functions, and at two different strata of language - generic structure and semantic organisation. A small quantum of research suggests that the dissociation between different strata of language (i.e., macro and micro linguistic abilities), identified in the spoken discourse of people with aphasia and people with cognitive-language disorder is mirrored in written discourse. Aphasic writers largely maintain coherent text structure while writers with cognitive-language impairment demonstrate problems with global text coherence and the episodic structure of texts. Results of the generic structure analysis did not support the hypothesis. However, the semantic Move analysis revealed how diminished linguistic resources, most evident in the letters written by the subjects with aphasia, impacted upon the semantic diversity of the text, as well as the interpersonal function of the personal letter. Variable performance as a feature pathology and normality is highlighted and clinical implications discussed.