Hemarthrosis leading to increased intracapsular pressure after an undisplaced femoral neck fracture is suspected to impair blood flow to the femoral head and may lead to osteonecrosis. We hypothesized that an increase of intraarticular pressure would reversibly decrease the blood flow to the femoral head. Eleven patients having surgical dislocations for treatment of femoroacetabular impingement were included in this study. Saline was injected into the intact intracapsular space while the blood flow to the femoral head was recorded using laser Doppler flowmetry. Injection of saline resulted in an increase in the intracapsular pressure. The injected volume of normal saline until disappearance of the pulsatile signal was on average 20 mL (range, 10–35 mL) with an average intraarticular pressure of 58 mm Hg (range, 25–88 mm Hg). Aspiration of the joint resulted in a return of the pulsatile flow. Hemarthrosis after undisplaced intracapsular fractures of the femoral neck are likely to cause an increase in intracapsular pressure. Decompression of the hematoma to reduce the intracapsular pressure is strongly advocated to optimize blood flow to the femoral head in acute fractures.