Family and twin studies have supported a strong genetic factor in the etiology of obsessive-compulsive disorder (OCD), although the precise mechanism of inheritance is unclear. Clinical and pharmacological studies have implicated the serotonergic and dopaminergic systems in disease pathogenesis. In this cross-sectional study, we have examined the allelic and genotypic frequencies of a Val-158-Met substitution in the COMT gene, a 44-base pair (bp) length variation in the regulatory region of the serotonin transporter gene (5-HTTLPR) and the T102C and C516T variants in the serotonin receptor type 2A (5HT2A) gene in 79 OCD patients and 202 control subjects. There were no observed differences in the frequencies of allele and genotype between patients and control groups for the COMT, the 5HTTLPR and the T102C 5HT2A gene polymorphisms. In contrast, a statistically significant difference between OCD patients and controls was observed on the genotypic distribution (chi(2) = 16.7, 2df, P = 0.0002) and on the allelic frequencies (chi(2) = 15.8, 1df, P = 0.00007) for the C516T 5HT2A gene polymorphism. The results suggest that the C516T variant of the 5HT2A gene may be one of the genetic risk factors for OCD in our sample. However, further studies using larger samples and family based methods are recommended to confirm these findings.