The study examined the relative efficacy of group acceptance and commitment therapy (ACT) for generalised anxiety disorder (GAD) compared to group cognitive-behavioural therapy (CBT). Fifty-one individuals with GAD were randomly allocated to a 6-week intervention, either ACT or CBT. Participants were assessed at pre-treatment, post-treatment, and 3-month follow-up on symptom measures, quality of life, and process measures. Data from 38 participants (19 in each group) were available at post-assessment, indicating significant improvements on all measures for both treatment conditions. Treatment gains were maintained at follow-up, with significant further improvements in anxiety, depression and stress symptoms for both groups. While no between-group differences were found at treatment completion, a significant interaction indicating steeper reduction in worrying symptoms (d = .79) from pre- to post-assessment was found for the ACT group compared to the CBT group. Furthermore, in relation to worrying, at treatment completion 78.9% of participants in the ACT group achieved reliable change compared to 47.4% of participants in the CBT group. However, both groups showed equivalent reliable change rates of 60% at the follow-up assessment. The results suggest that group ACT was as efficacious as group CBT. While participants in the ACT group maintained treatment gains at follow-up, participants in the CBT group continued to improve between post-assessment and follow-up.