This study investigated whether an analysis of narrative style (word use and cross-clausal syntax) of patients with symptoms of generalised anxiety and depression disorders can help predict the likelihood of successful participation in guided self-help. Texts by 97 people who had made contact with a primary care mental health service were analysed. Outcome measures were completion of the guided self-help programme, and change in symptoms assessed by a standardised scale (CORE-OM). Regression analyses indicated that some aspects of participants' syntax helped to predict completion of the programme, and that aspects of syntax and word use helped to predict improvement of symptoms. Participants using non-finite complement clauses with above-average frequency were four times more likely to complete the programme (95% confidence interval 1.4 to 11.7) than other participants. Among those who completed, the use of causation words and complex syntax (adverbial clauses) predicted improvement, accounting for 50% of the variation in well-being benefit. These results suggest that the analysis of narrative style can provide useful information for assessing the likelihood of success of individuals participating in a mental health guided self-help programme.