We dissected 30 cadaveric knees to provide a detailed anatomic description of the biceps femoris muscle complex at the knee. The main components of the long head of the muscle are a reflected arm, a direct arm, an anterior arm, and a lateral and an anterior aponeurosis. The main components of the short head of the biceps femoris muscle are a proximal attachment to the long head's tendon, a capsular arm, a confluens of the biceps and the capsuloosseous layer of the iliotibial tract, a direct arm, an anterior arm, and a lateral aponeurosis. We examined 82 consecutive, acutely injured knees with clinical signs of anterolateral-anteromedial rotatory instability for the incidence and anatomic location of injuries to the biceps femoris muscle. Injuries to components of that muscle were identified in 59 (72%) of these knees; 29 knees (35.4%) had multiple components injured. There were 3 injuries to the long head of the biceps femoris muscle (all in the reflected arm) and 89 to the short head. A statistically significant correlation (P = 0.01) was found between increased anterior translation with the knee at 25 degrees of flexion as demonstrated by the Lachman test and injury to the biceps-capsuloosseous iliotibial tract confluens. Additionally, adduction laxity at 30 degrees of flexion correlated with a Segond fracture (P = 0.04). These data establish, in part, the relationship of the biceps femoris complex injury to anterior translation instability.