To study the temporal course and the relationship between autonomic failure and motor weakness in Guillain-Barré syndrome (GBS), a total of 164 autonomic function tests were serially applied for up to 1 year in 13 consecutive patients. Results were compared with those obtained in 25 healthy volunteers and 13 patients with other neurological diseases. Parasympathetic function tests comprised heart rate responses to Valsalva maneuver, deep breathing, and active change of posture, whereas sympathetic vasomotor function was assessed by blood pressure responses to handgrip and standing. At the height of the disease, subclinical autonomic involvement of both parasympathetic and sympathetic arms was revealed in the vast majority of patients. Abnormalities of autonomic function tests improved gradually over time, paralleling the recovery of motor function, but were only partially related to clinically significant autonomic dysfunction. Correlation analysis suggested that tachycardia in the context of GBS might be caused by a reduction of sympathetically mediated peripheral vascular tone rather than by vagal failure.