Background/Aims: The aim of this study was to investigate the serotonin transporter (SERT), by means of the <sup>3</sup>H-paroxetine ([<sup>3</sup>H]-Par) binding to platelet membranes, in patients affected by pathological gambling (PG), as compared with a similar group of healthy control subjects. Methods: Seventeen PG patients were selected amongst those who were drug-free and at the first psychiatric interview in a Department of Addiction. The diagnosis was assessed according to DSM-IV criteria and PG severity was measured by means of the South Oaks Gambling Screen. The platelet [<sup>3</sup>H]-Par binding was carried out according to a standardized method. The binding parameters, the maximum binding capacity (B<sub>max</sub>) and the dissociation constant (K<sub>d</sub>), were obtained by means of the Scatchard analysis. Results: The B<sub>max</sub> values of PG patients were significantly lower than that of healthy subjects, while the K<sub>d</sub> values were not different in the two groups. No significant effect of age, sex or psychiatric comorbidity on B<sub>max</sub> or K<sub>d</sub> was observed; there were also no correlations between clinical and biological variables. Conclusions: PG patients showed a dysfunction at the level of the platelet SERT that would suggest the involvement of the 5-HT system in this condition.