To elucidate the cause of idiopathic rapid eye movement (REM) sleep behavior disorder (RBD), magnetic resonance imaging and single-photon emission computed tomography of the brain were conducted on 20 patients with RBD. Blood flow in the upper portion of both sides of the frontal lobe and pons was significantly lower in patients with RBD than in the normal elderly group. Among the patients with RBD, decreased blood flow in the frontal lobe showed no correlation with the extent of frontal lobe atrophy. Decreased blood flow in the upper portion of the frontal lobe and pons might be associated with the pathogenesis of idiopathic RBD.