Fifteen TMJS of fifteen normal controls and thirty TMJs of thirty patients with TMJDS were studied with head-coil MR imaging. T1 weighted oblique sagittal scanning in closed and opened mouth and T2 weighted oblique sagittal scanning in closed mouth were used to evaluate the lateral pterygoid muscle of the controls and the patients. The upper head of lateral pterygoid muscle was found shorter and straighter in opened mouth than that in closed mouth. It is suggested that the upper head is in the state of contraction in opened mouth. The same finding was also found in the patients with TMJDS in opened mouth. Edema and effusion in lateral pterygoid muscle were found in 80% patients with TMJDS on T2 weighted image.