The electrogastrogram (EGG) was recorded for 24 hours in 17 Parkinson's disease (PD) patients, 17 multiple system atrophy (MSA) patients, and 8 healthy control subjects to elucidate the differences in the EGG findings between the two diseases. Eight EGG segments (3 preprandial, 3 postprandial, and 2 sleep segments) were selected from the total recording for spectral analysis, from which we obtained the dominant frequency (DF), instability coefficient of DF (ICDF), and low (LFR%), normal (NFR%), and high (HFR%) range power percentages of the total power. PD patients showed irregular slow waves, high HFR%, and high ICDF, whereas MSA patients showed regular slow waves and low ICDF. Although DF and NFR% increased after meal in controls, postprandial increases in DF and NFR% were less significant in both patient groups compared to the controls. The PD patients presented gastric dysrhythmias indicating gastric pacemaker disturbances. The MSA patients showed regular slow waves with low variability of the slow wave rhythm (low ICDF), which might have resulted from the involvement of gastric autonomic nerve function. © 2009 Movement Disorder Society