Pharmacological studies have shown that sumatriptan, a selective ligand of the serotonin 5-HT-1Dβ autoreceptor, modifies obsessive–compulsive disorder (OCD) symptoms. The current study analysed the G861C polymorphism of the 5-HT-1Dβ gene in a sample of 72 trios. Genotyping data were analysed using the Family-Based Association Test (FBAT). We did not replicate the previously reported linkage disequilibrium between the G861 variant and OCD. However, a quantitative trait analysis, assessing severity of OCD symptoms and defined as YBOCS score, confirmed the finding that subjects with a preferential transmission of the G861 variant showed higher YBOCS Obsession scores compared to patients carrying the C861 allele. These preliminary findings may indicate that the 5-HT-1Dβ receptor gene could be involved in the severity of obsession symptoms in OCD. However, it is important to perform the replication of these findings in larger sample sizes of informative families.