Treatment with acetyl-leucine led to a marked decline in the clinical severity of the RBD phenotype (disappearance of aggressive dream content, reduction of dream enactment); a stabilization or reversal of the slow, progressive decline of specific striatal DAT-SPECT binding ratios; and a stabilization or reversal of the expression of the PDRP, an indicator of the progressive metabolic impairment in the CNS of iRBD and PD—as measured by FDG-PET in two patients with REM Sleep Behavior Disorder.