A 28-year-old major league baseball pitcher sustained an axillary artery thrombosis which was successfully treated with intraarterial urokinase. Subsequent angiography and duplex scanning with the arm elevated in the pitching position demonstrated inducible compression of the axillary artery by the humeral head as well as compression at the thoracic outlet. To determine the incidence of axillary and subclavian artery compression and to investigate the mechanism of injury, brachial artery blood pressures and duplex scans of the subclavian and axillary arteries were performed in both the neutral position and the throwing position in the 92 extremities of 19 major league baseball pitchers, 16 non-pitching major league players, and 11 nonathlete controls. A drop in blood pressure of greater than 20 mm Hg was noted in the position in 56% of extremities tested, with a loss of a detectable blood pressure in 13%. Compression of the axillary artery by the humeral head was documented in 83% of extremities, but in only 7.6% was a greater than 50% stenosis inducible. No statistical difference was found in the incidence of arterial compression between the three groups tested or between their dominant and nondominant extremities. Dissection of the axillary artery in two cadavers documented that abduction and external rotation of the arm causes compression of the axillary artery by the humeral head, which acts as a fulcrum. We conclude that the repetitive mechanical trauma of the throwing motion can cause intermittent compression and contusion of the axillary artery by the humeral head and predisposes the athlete who throws to thrombosis of the axillary artery.