Objective Weight loss and behavioral disturbances are frequent over the course of Alzheimer's disease (AD) and are risk factors for poor outcome. We investigated the impact of aberrant motor behavior (AMB) on weight changes in older adults with AD. The hypothesis that patients with AMB are more likely to lose weight than patients without AMB was assessed. Methods A prospective study of 686 patients with moderate AD from the REAL.FR cohort was assessed. The AMB at baseline was defined by the item 10 from the Neuropsychiatric Inventory scale (NPI‐10). Patients were classified as “no or light AMB” (NPI‐10 < 4), and “significant AMB” (NPI‐10 ≥ 4). Weight changes were determined over the 4‐year follow‐up. Results The mean weight change over the 4 years was +2.2 ± 0.9 kg in patients with “significant AMB,” whereas weight remained stable in patients with “no or light AMB” (p = 0.02). Conclusion Older adults with moderate AD and “significant AMB” do gain weight. Copyright © 2012 John Wiley & Sons, Ltd.