In a group of 45 patients with Alzheimer disease (AD) and 15 control subjects, we studied the relationship between parameters showing postural and gait disturbance and regional cerebral blood flow (rCBF) measured with N-isopropyl-p-[123I]iodoam-phetamine single photon emission computed tomography at different clinical stages. Patients with AD with mild dementia exhibited only increased postural sway associated with a reduced mean value of rCBF in the cortex. At a moderate stage, the reduced mean values of rCBF in the cortex and in the frontal lobe were associated with increased postural sway and stride length variability and with decreased stride length. At a severe stage, reduced rCBF in the basal ganglia and in the frontal lobe additionally were associated with increased postural sway, double support time, and stride length variability, and with decreased walking speed and stride length. In multiple regression analysis, there was a high contribution by rCBF in the frontal lobe to account for postural and gait disturbance in AD. We propose that impaired frontal lobe circulation is an important factor causing postural and gait disturbance as AD progresses.