Background: Upper extremity vascular injuries are uncommon in the elite throwing athlete. However, the extreme stresses that are placed on the upper extremity of elite baseball players, especially pitchers, puts them at risk for such injuries. One such injury is upper extremity venous thrombosis or "effort thrombosis." Purpose: We wanted to review the common initial clinical symptoms and physical examination findings of effort thrombosis in elite baseball players and to review the associated clinical conditions such as hypercoagulable states and pulmonary embolism. Study Design: Retrospective review of a series of cases. Methods: A retrospective review of the medical records of a Major League Baseball organization and a Division I college was performed for the period 1987 to 1997. Results: We located four cases of effort thrombosis involving elite baseball players. Contrast venography was used to confirm the diagnosis in all cases. All patients were successfully treated with transluminal catheter-directed urokinase thrombolysis followed by first rib resection and systemic anticoagulant therapy for up to 3 months. All four players returned to play at or above their previous level of competition with no long-term chronic sequelae. Conclusions: Prompt clinical recognition, diagnosis, and treatment of effort thrombosis in the elite baseball player provides the player with an excellent prognosis for return to the previous level of play.