Established treatments for obsessive compulsive disorder (OCD) include cognitive behaviour therapy (CBT) and selective serotonin reuptake inhibitor (SSRI) medication. Combined treatment may outperform monotherapy, but few studies have investigated this. A total of 49 community-based adults with OCD were randomly assigned to CBT, SSRI, or SSRI+CBT. Sertraline (50-200 mg/day) was given as the SSRI for 52 weeks. A 16-h-manualized individual CBT was delivered over 8 weeks with four follow-up sessions. Assessors were 'blinded' to treatment allocation. A preliminary health economic evaluation was conducted. At week 16, combined treatment (n=13) was associated with the largest improvement, sertraline (n=7) the next largest and CBT (n=9) the smallest on the observed case analysis. The effect size (Cohen's d) comparing the improvement in Yale Brown Obsessive Compulsive Scale on CBT versus combined treatment was -0.39 and versus sertraline was -0.27. Between 16 and 52 weeks, the greatest clinical improvement was seen with sertraline, but participant discontinuation prevented reliable analysis. Compared with sertraline, the mean costs were higher for CBT and for combined treatment. The mean Quality Adjusted Life Year scores for sertraline were 0.1823 (95% confidence interval: 0.0447-0.3199) greater than for CBT and 0.1135 (95% confidence interval: -0.0290-0.2560), greater than for combined treatment. Combined treatment appeared the most clinically effective option, especially over CBT, but the advantages over SSRI monotherapy were not sustained beyond 16 weeks. SSRI monotherapy was the most cost-effective. A definitive study can and should be conducted.