['Postural tachycardia syndrome (POTS) is related to defective peripheral vasoconstriction of dependent extremities with redistributive hypovolemia.', 'To test whether enhanced microvascular filtration produces leg enlargement, we studied 12 patients 13 to 19 years of age with POTS and defective leg vasoconstriction and 13 age-matched healthy control subjects, with strain-gauge plethysmography used to measure venous pressure (Pv), forearm and calf blood flow, vascular capacitance, and the microvascular filtration coefficient (Kf). Measurements were made while the patient was supine and at steady state during upright tilt to 35 degrees. Supine Pv was not different in POTS, but upright leg Pv tended to be increased above control. Arm and leg peripheral arterial resistance was decreased in the supine and upright positions in patients with POTS compared with control subjects (P=0.01, upright legs). Supine Kf was not significantly increased in the forearm in patients with POTS but was increased in the calf (9.3+/-2.2 versus 5.7+/-2.4 [10(-3)] mL/100 mL per minute per mm Hg, P=0.04), correlating with calf blood flow (rs=0.84, P=0.002). Kf was invariant with orthostasis. The hydraulic contribution to upright filtered flow at 35 degrees tilt, the product of Kf and Pv, was approximately twice that of control (0.41+/-0.09 versus 0.19+/-0.04 mL/100 mL per minute, P=0.04).'] Is microvascular filtration increased in postural tachycardia syndrome?