OBJECTIVE: Serotonin abnormalities may be involved in the etiopathogenesis of obsessive-compulsive disorder (OCD). The silent G-to-C substitution at nucleotide 861 of the coding region of the 5-HT1Dβ receptor gene may be associated with liability to OCD. The aim of this study was to investigate this association in an Italian OCD study group. METHOD: Genotyping for 5-HT1Dβ was performed for 79 nuclear families of probands with OCD. The transmission/disequilibrium test was used to determine transmission of the alleles from parents to offspring. RESULTS: Of the 79 families, 48 were informative for the analysis, i.e., both parents were genotyped for 5-HT1Dβ, and at least one parent was heterozygous. No preferential transmission of either allele of the 5-HT1Dβ gene was observed. CONCLUSIONS: These data do not support a role for the 5-HT1Dβ receptor gene in conferring susceptibility to OCD.