Children usually improve their writing in response to teacher comments. HARRY is a computer tutor, designed to assist children improve their narrative writing, focusing particularly upon grammar and style. Providing assistance involved identifying aspects of grammar and style on which to concentrate, including ways to enable the computer to detect weaknesses and then present information concerning how to make improvements. HARRY delivers general, conversational style suggestions for how to revise a text during composition, followed by editing suggestions targeting specific grammar and style weaknesses detected in the text. Finally, spellings and technical errors are corrected in MS Word. Evaluating a writing intervention strategy is problematic, as paradoxically, attempts by children to improve texts, can result in a deterioration in some aspects of writing quality. Four case studies are presented to demonstrate the effects of the HARRY writing system upon children's grammar and style. Each child wrote a control and a HARRY assisted story. HARRY's effectiveness at improving written grammar and style is demonstrated through analyses made of the narratives by CHECK TEXT, a utility tool that calculates and interprets quantitative data. Separate analysis of each stage of the HARRY assisted stories reveals the idiosyncratic pattern of improvement exhibited by each child.