We investigated the association between the long (l) and short (s) alleles of the serotonin transporter polymorphism (5-HTTLPR) in the promoter region of the SLC6A4 gene and obsessive-compulsive disorder (OCD) using meta-analysis to combine all published data from case–control and family based association studies (2,283 cases). In stratified meta-analysis we investigated whether age of sample (child and adult), ethnicity (Caucasian and Asian) and study design (case–control and family-based association studies) moderated any association. In the overall meta-analysis we found no evidence of association between genetic variation at the 5-HTTLPR locus and OCD. We did find significant heterogeneity between studies. In the stratified meta-analyses, we demonstrated a significant association between the l-allele and OCD in family-based association studies and in studies involving children and Caucasians. Our meta-analysis suggests the possibility that the l-allele may be associated with OCD in specific OCD subgroups such as childhood-onset OCD and in Caucasians. Further meta-analyses based on individual patient data would be helpful in determining whether age of OCD onset, gender and the presence of comorbid illness (e.g., tics) moderates the relationship between 5-HTTLPR and OCD. © 2008 Wiley-Liss, Inc.