A 28-yr-old man, diagnosed with vascular thoracic outlet syndrome, had his right anterior scalene muscle injected with botulinum toxin type A under ultrasound guidance. Subclavian artery flow rates were measured with Doppler ultrasound before and 3 wks after the injection. At 3 wks, symptoms had improved. The reduction in subclavian artery flow rate associated with the arm abducted to the side with 90-degree external rotation decreased from 132.8 to 87.7 cm/sec preinjection compared with a smaller decrease postinjection from 132.8 to 119.1 cm/sec. An improved subclavian artery flow was seen after scalene muscle chemodenervation for vascular thoracic outlet syndrome. This suggests its potential as a diagnostic test, treatment option, and predictor for surgical intervention.