Background - Several studies have found a heightened prevalence of mental health disorders in people with intellectual disabilities (ID). There have been a number of successful case series and two promising controlled treatment trials of cognitive behaviour therapy (CBT) for emotional disorders (excluding anger) for people with ID. Several authors have promoted the development of trans-diagnostic approaches to cognitive treatment. The present study extends this work with the development and evaluation of a trans-diagnostic treatment manual for CBT in people with ID. Method - A controlled treatment trial was conducted with 12 participants in treatment and waiting list control data. Each treatment participant was matched to a control on age, IQ, presenting problem, and Brief Symptom Inventory (BSI) global severity index (GSI) score. The treatment group was also evaluated on the Glasgow anxiety and depression scales and was followed up for 3 to 6 months after treatment. Results - There were no significant differences between groups at baseline. Following treatment, the CBT group was significantly improved when compared with the control group on the GSI scale of the BSI. The ancovas for all other measures were not significant but there were significant improvements for the treatment group on all scaled except BSI depression from pre to post-CBT. Gains were maintained to follow up, and changes were associated with large effect sizes. Conclusions - It was possible to treat a range of symptoms and psychiatric diagnoses with a general trans-diagnostic CBT manual. The effects of therapy were promising, suggesting that the participants could respond to treatment in a meaningful and helpful manner and supporting the case for further evaluation of the trans-diagnostic approach in ID.