In a prospective clinical study the intraarticular pressure of 55 patients with intracapsular femoral neck fractures was measured intraoperatively with the hip in different positions. Intraarticular hemarthrosis was quantified by a preoperative sonography examination. In 75% of the patients, increased intraarticular pressure caused by the hemarthrosis was found. The spontaneous median pressure increased significantly from 22 mm Hg with extension (28 mm Hg) and internal rotation of the hip joint (56 mm Hg). The lowest pressure was found in 70 degrees flexion (15 mm Hg). The median pressures increased within the first 24 hours after injury from 26 mm Hg in the first 6 hours to 46 mm Hg from 7 to 24 hours. Even in the first and second weeks after trauma, increased median pressures were detected (8.5 mm Hg and 13 mm Hg, respectively). No significant difference was found between undisplaced and displaced fracture types. Because increased joint pressure in other studies correlates with reduced perfusion of the femoral head, it can be deduced that reduction maneuvers without capsulotomy can compromise the circulation of the femoral head. Capsulotomy and osteosynthesis of the femoral neck at the earliest time possible is the best prophylaxis of tamponade. If the osteosynthesis is delayed, a preoperative sonography after admission and a control sonogram after 6 hours is recommended. In the event of relevant hemarthrosis, immediate therapeutic drainage is suggested for patients who will receive joint conserving osteosynthesis.