Background: Very few studies have compared the efficacy of individual and group cognitive behaviour therapy (CBT) for the treatment of Obsessive-Compulsive Disorder (OCD) by taking into consideration the change in OCD severity in both the short and long term. Aims: To conduct an open trial of individual versus group CBT for OCD, comparing the clinical and statistically significant changes in severity both at post-treatment and one year later. Method: Forty-two OCD subjects were assigned to individual (n = 18) or group CBT (n = 24, in four groups). Sixteen and 22 subjects completed the treatment in the individual and group conditions, respectively. The Yale-Brown Obsessive Compulsive Scale was recorded at pre-treatment, post-treatment and at the one-year follow-up. Results: At the end of treatment, the clinically significant change was comparable for the two treatment conditions and remained stable at the one-year follow-up. Of the 16 participants who completed the individual CBT treatment, 68.75% were classified as recovered at post-treatment, compared to 40.9% of those receiving group CBT. At follow-up the rate of recovery decreased to 62.5% in individual CBT and to 31.8% in group CBT. Conclusions: Group CBT is effective in decreasing OCD severity. The post-treatment changes were maintained one year later. Nevertheless, these changes were higher in the individual delivery of CBT.