Bone changes in the mandibular head and mandibular fossa in 33 patients with TMJ disorders were studied with axial computed tomography in relation to clinical findings to clarify possible factors leading to bone changes in this phenomenon. Bone changes of the mandibular head were observed in 45 (68%) of the 66 TMJs. The mandibular head was juxtaposed to the mandibular fossa in 13 (29%) of the 45 joints in centric occlusion and in 29 joints (64%) in the anterior position on CT, whereas the mandibular head with no pathological bone change was juxtaposed to the mandibular fossa in none of the 21 joints in centric occlusion and in only 1 joint (5%) in the anterior position. In the two groups of patients with and without juxtaposition of the mandibular head and mandibular fossa with bone changes, the incidence of the initial symptoms such as pain. crepitus, and difficulty in opening the mouth was increased compared with the symptoms at presentation. However, the former group had severer symptoms than the latter group. These findings suggest that bony degeneration of the TMJ is accelerated by juxtaposition of the head and fossa.