Platelet serotonin uptake was measured in 45 patients (12 males, 33 females) with panic attacks and 21 controls (9 males, 12 females). Higher Vmax values were obtained in the patient group than in the controls (100.2 +/- 11.5 vs. 34.9 +/- 3.8 pmol/10(8) platelets/min; P less than 0.0005) while the affinity constant Km was not significantly different (2.28 +/- 0.3 vs. 1.71 +/- 0.18 microM). A value of Vmax in excess of 60 pmol/10(8) platelets/min was observed in 60% of patients and in only 5% of controls. The results point to a specific abnormality of platelet serotonin uptake in patients with panic attacks.