The efficacy of group cognitive behaviour therapy (CBT) for obsessive-compulsive disorder (OCD) has received relatively little research attention compared with the large number of studies that have investigated individual CBT. The current study is the first controlled study to compare an identical CBT protocol, containing both cognitive and behavioural elements, delivered either individually or in a group. Participants were randomly assigned to either 10 weeks of individual CBT, 10 weeks of group CBT or a 10 week wait-list. Participants with significant rates of secondary comorbidity were included in the study to enhance the generalisability of results. Intention-to-treat and completer analyses were carried out and indicated no differences between the group and individual treatments on outcome measures. Large effect sizes were found for both conditions. Analysis of clinically significant change indicated that the individual treatment was associated with a more rapid response but that both treatments had equivalent rates of recovered participants by brief follow-up. The importance of further investigations of the efficacy of group CBT for OCD is discussed.