BACKGROUND: Single case studies are a valuable means of providing information about the outcomes of speech and language intervention. Many previous studies have used phonological analysis as a theoretical basis, while others have used psycholinguistic models. The present study combines these approaches to assessment, intervention and evaluation of outcomes. AIMS: The aim of the research was to determine if intensive psycholinguistically based intervention could result in (a) specific and (b) generalized improvements in the speech production of a child with severe and persisting phonological difficulties. METHODS AND PROCEDURES: A single subject research design was used with pre- and post-intervention assessment carried out. Assessment took place at two levels: the macro level focused on global change in the child's speech processing system using psycholinguistic speech profiling and phonological analyses; the micro assessment focused on specific, treated (and matched control) stimuli. There were three phases of intervention with a total of 30 hours of therapy. OUTCOMES AND RESULTS: Micro evaluation showed significant changes in Katy's single word and connected speech production-as well as in other areas such as spelling and auditory discrimination of closely related real word pairs. Macro evaluation revealed significant improvement in speech severity indices (PCC, PPC), and gains in her ability to discriminate between closely related real word pairs. These changes were maintained at follow-up seven months after intervention had ceased. CONCLUSIONS: Specific and intensive intervention brought about significant improvements in this child's speech, spelling and auditory discrimination at the micro level, and in speech and auditory discrimination at a macro level. The macro level assessments reveal persisting core deficits requiring further focused intervention before changes can be observed more widely. When carefully defined and evaluated, speech and language intervention can have positive outcomes for children with severe and persisting speech difficulties.