Avascular necrosis is the death of trabecular bone and bone marrow and is most common in the femoral head. The pathogenesis is believed to result from ischemia caused by intraosseous microcirculation impairment. The aim of the treatment is to prevent the collapse of the femoral head and thus to stop the development of osteoarthritis. Hyperbaric oxygen therapy acts by aiming to correct the underlying pathophysiology of avascular necrosis of the femoral head. This treatment involves supplying 100% oxygen to the whole body at pressures higher than atmospheric pressure. Hyperbaric oxygen therapy causes vasoconstriction in the arterioles, reducing inflammation and promoting collagen and blood vessel synthesis (angiogenesis). It also provides direct oxygenation of the necrotic region in the trabecular bone via the extracellular fluid. As a result of all these, venous pressure decreases in the femoral head and microcirculation improves. Thus clinically, pain is significantly reduced and joint movements are increased. Hyperbaric oxygen therapy can be applied in patients diagnosed with avascular necrosis in the early period (stage I-II) alone or together with other treatments (cocktail therapy) instead of only observation.