BACKGROUND: Awareness of individual phonemes in words is a late-acquired level of phonological awareness that usually develops in the early school years. It is generally agreed to have a close relationship with early literacy development, but its role in speech change is less well understood. Speech and language therapy for children with speech disorder involves tasks that appear, either implicitly or explicitly, to require a phonemic level of awareness. However, children typically attend for intervention at a pre-school, pre-literate stage, i.e. before they would be expected to have developed the relevant phoneme segmentation and manipulation skills. AIMS: To investigate whether it is possible to teach phoneme awareness skills to pre-literate children with speech disorder. METHODS & PROCEDURES: In a randomized controlled trial design 42 children with speech disorder, aged 4;0-4;6, were allocated to either a phonological awareness or a language stimulation programme. Children were assessed on four measures of phoneme awareness (alliteration awareness, phoneme isolation, word segmentation and phoneme addition/deletion) immediately before and after the programme and categorized as 'improved' or 'not improved' according to predetermined criteria. Fisher's Exact test was used to compare outcome in the two groups. OUTCOMES & RESULTS: Significantly more children improved in the phonological awareness group than in the language stimulation group for three out of the four measures (all except alliteration awareness). However, for the two most advanced tasks (segmentation and addition/deletion) only a small minority of children showed improvement. A marked improvement in Phoneme Isolation was made by the majority of children in the phonological awareness group. CONCLUSIONS: It is possible to teach some advanced phoneme awareness skills to some pre-literate children. Phoneme Isolation was the most easily learned and is a skill that appears very relevant to speech and language therapy. However, phoneme addition, deletion and word segmentation showed relatively limited improvement and only in a small number of cognitively able and older children. Whereas isolation of word initial consonants appears to be a skill that can be triggered at 4;0-4;6 by relevant activities, most children in the study were not cognitively ready for more advanced, abstract phoneme manipulation tasks. This raises questions about how speech and language therapists should tackle many common errors and the age at which we should aim to develop or draw on phoneme awareness to stimulate speech change.