We describe an unrecognized mechanical condition affecting the long head of the biceps (LHB) tendon with entrapment of the tendon within the joint and subsequent pain and locking of the shoulder on elevation of the arm. We identified 21 patients with a hypertrophic intraarticular portion of the LHB tendon during open surgery (14 patients) or arthroscopic surgery (7 patients). All cases but one were associated with a rotator cuff rupture. Patients were treated by biceps tenotomy (2 patients) or tenodesis (19 patients) after removal of the hypertrophic intraarticular portion of the tendon and appropriate treatment of concomitant lesions. Minimum follow-up was 1 year. All patients presented with anterior shoulder pain and loss of active and passive elevation averaging 10 degrees to 20 degrees. A dynamic intraoperative test, involving forward elevation with the elbow extended, demonstrated entrapment of the tendon within the joint in each case. This test creates a characteristic buckling of the tendon and squeezing of it between the humeral head and the glenoid (hourglass test). The mean Constant score improved from 38 to 76 points at the final follow-up (P <.05). Complete and symmetric elevation was restored in all cases after resection of the intraarticular portion of the LHB tendon. The hourglass biceps is caused by a hypertrophic intraarticular portion of the tendon that is unable to slide into the bicipital groove during elevation of the arm; it can be compared with the condition of trigger finger in the hand. A loss of 10 degrees to 20 degrees of passive elevation, bicipital groove tenderness, and radiographic findings of a hypertrophied tendon can aid in the diagnosis. A definitive diagnosis is made at surgery with the hourglass test: incarceration and squeezing of the tendon within the joint during forward elevation of the arm with the elbow extended. The hourglass biceps is responsible for a mechanical block, which is similar to a locked knee with a bucket-handle meniscal tear. Simple tenotomy cannot resolve this mechanical block. Excision of the intraarticular portion of the LHB tendon, during bipolar biceps tenotomy or tenodesis, must be performed. The hourglass biceps is an addition to the familiar pathologies of the LHB (tenosynovitis, prerupture, rupture, and instability) and should be considered in cases of shoulder pain associated with a loss of elevation.