Peripheral benzodiazepine (BDZ) receptors were investigated through the binding of the specific ligand 3H-PK-11195 to platelet membranes, in 17 patients suffering from panic disorder (PD) and in 16 patients affected by obsessive-compulsive disorder (OCD). The results, showing that the density (Bmax) of peripheral BDZ receptors was significantly lower in patients with PD than in controls or OC patients, suggest that the number of platelet BDZ receptors varies with different anxiety disorders and that perhaps this marker may be beneficial in differentiating some subtypes of these disorders.