Exertional compartment syndrome in the deep posterior compartment of the leg is well recognized. This paper reports investigations which were performed to prove that the tibialis posterior muscle is contained in its own osseofascial compartment, separate from the rest of the deep posterior compartment. Radiographs following the injection of radio-opaque dye into the tibialis posterior muscle demonstrated the fluid-impermeable, osseofascial boundaries surrounding this muscle. Compartment syndromes created in cadaver legs reveal that traditional techniques of fasciotomy of the deep posterior compartment are inadequate in decompressing the tibialis posterior muscle. Intracompartmental pressure measurements in athletes with a clinical diagnosis of exertional compartment syndrome proved the existence of isolated exertional compartment syndrome in the tibialis posterior muscle. From these results we may conclude that the tibialis posterior muscle is contained in a separate osseofascial muscle compartment which may be the site of an isolated exertional compartment syndrome and that common techniques of fasciotomy of the deep posterior compartment do not decompress the tibialis posterior muscle.