Objective The aim of this study was to compare efficacy of fluoxetine alone and co-administration of gabapentin and fluoxetine in patients with obsessive compulsive disorder (OCD). Methods Forty outpatients with a DSM-IV diagnosis of OCD were randomized to open label treatment, 20 of whom were treated with fluoxetine alone and the remaining 20 with fluoxetine plus gabapentin during 8 weeks. The severity was assessed by Yale-Brown Obsessive Compulsive Scale (Y-BOCS) and Clinical Global Impression (CGI). Results Final CGI-I and Y-BOCS scores were not significantly different in both groups. However, in repeated measures ANOVA, compared to fluoxetine group, we found significantly a better improvement in the fluoxetine plus gabapentin group at week 2 by means of YBOCS and CGI-I scores. Comparisons on weeks 4, 6 and 8 revealed no statistical differences between the groups. There was no significant difference of adverse effects between two groups. Conclusions Adding gabapentin to fluoxetine in the treatment of OCD seems to shorten the time to onset of fluoxetine's anti-obsessive effect without a significant increase in adverse effects. In order to accelerate the clinical response, co-administration of fluoxetine and gabapentin may be a preferable strategy. On the other hand, further controlled studies are needed to support this finding.