An increasing number of cognitive neuropsychological treatment studies of acquired dysgraphia have been published in recent years, but to our knowledge there are no corresponding studies of developmental dysgraphia. This paper reports a cognitive neuropsychological treatment programme designed for a child with developmental surface dysgraphia. The treatment aim was to improve functioning of the orthographic output lexicon, and so treatment methods targeted irregular word spelling. Treatment methods were based on previous successful treatments employed in cases of adult acquired surface dysgraphia (Behrmann, 1987; De Partz, Seron, & Van der Linden, 1992; Weekes & Coltheart, 1996). Results showed a significant treatment effect for both spelling and reading of irregular words that was largely stable over time and that generalised partially to spelling of untreated irregular words. Homophone words were not treated but some aspects of homophone reading and spelling also improved, though homophone confusion errors remained. Comparison of treatment effectiveness with and without mnemonics suggested that the mnemonic cue itself was not necessary to achieve treatment success for irregular word spelling. Analyses revealed that untreated irregular words whose spellings became correct as a result of treatment generalisation were those whose original misspellings were closest to being correct prior to treatment. Results also provided preliminary evidence that the mechanism underlying treatment generalisation involved improved access to orthographic representations, resulting in an increased tendency to employ orthography for spelling attempts and reduced reliance on phoneme to grapheme conversion.