This study investigated the effects of group cognitive behavioural therapy (CBT) for patients with bipolar disorder. The development of CBT for this disorder is relatively under-explored.Method: Participants with bipolar I or II disorder were treated with group CBT in addition to treatment as usual. The effectiveness of the protocol was explored through sequence analysis of daily mood monitoring prior to, during and after the intervention. Also, a repeated measures design was used assessing symptomatology, dysfunctional attitudes, sense of mastery, psychosocial functioning, and quality of life at start and end of intervention, and at follow-up 2 and 12 months later.Results: The results indicate that variation in mood states diminished over the course of the intervention. Also, there was a change from depressive states to more euthymic states. Greater number of reported lifetime depressive episodes was associated with greater diversity of mood states. There was an increase in overall psychosocial functioning and self-reported psychological health following the intervention. Improvement continued after treatment ended until follow-up at 2 months, and measured 1 year later, for outcomes representing depression, general psychosocial functioning and self-reported psychological health. Due to small sample size and the lack of a control group the results are preliminary.Conclusions: The results of this pilot study suggest that both offering CBT in group interventions and sequence analysis of time series data are helpful routes to further explore when improving standard CBT interventions for patients suffering from bipolar disorder.