Several trials have indicated that selective serotonin reuptake inhibitors (SSRIs) are most efficient in the treatment of social phobia (SP). The activity of the serotonin transporter (5-HTT), as determined by [3H]5-HT uptake to blood lymphocytes, was measured in 15 drug-free patients with generalized SP (7M/8F, aged 21-37 years) and compared to 18 healthy control subjects (10M/8F, aged 21-32 years). The maximum uptake velocity (Vmax) of [3H]5-HT to fresh lymphocytes and the affinity (Km) of the 5-HTT were similar in the two groups (295+/-155 versus 227+/-117 pmol/min/mg protein and 767+/-425 versus 709+/-408 nM, respectively). This study suggests that the functioning of lymphocyte 5-HTT is unaltered in SP.