We have determined the pressure-flow relationships of the canine femoral head during venous tamponade of the hip capsule. Intra-osseous pressures were determined before and after infusion of the joint with saline solution to a pressure of sixty-five centimeters of water. Femoral head blood flow was simultaneously determined by the indicator-dilution technique utilizing isotopically labeled microspheres. In puppies, pressure in the femoral head rose 248 per cent while flow dropped by 60 per cent after inflation of the capsule. In adults, no statistically significant change in either pressure or flow was seen. Thus, in the immature animal, venous tamponade results in increased pressure and decreased blood flow. In the mature animal, venous tamponade does not alter intra-osseous pressures or blood-flow rates due to the intact intramedullary venous drainage of the adult femoral head. CLINICAL RELEVANCE: In the immature individual, venous tamponade may well be involved in the development of Legg-Perthes disease. A bout of nonspecific synovitis may elevate intracapsular pressure sufficiently to obstruct venous outflow. This creates an increase in intra-osseous pressure and a decrease in femoral head blood flow. Hemodynamic changes of this magnitude have not been shown to induce Legg-Perthes disease; however, a strong suspicion exists that such alterations may be linked to the disease. In the adult, venous tamponade probably is not involved in the pathogenesis of avascular necrosis. The maintenance of intramedullary venous drainage of the epiphysis into the metaphysis may account for the fact that vascular necrosis of the femoral head rarely develops in adult patients with osteoarthritis or rheumatoid arthritis plus an inflammatory synovitis of the hip.