Technical Abstract: Patients with Parkinson's disease (PD) commonly exhibit weight loss (WL) which investigators attribute to various factors, including elevated energy expenditure. We tested the hypothesis that daily energy expenditure (DEE) and its components, resting energy expenditure (REE) and physical activity (PA) energy expenditure (PAEE), are elevated in WL compared with weight stable (WS) PD patients. We measured DEE in 10 PD WL patients and 10 PD WS patients using doubly labeled water (DLW). PAEE was estimated with DLW, activity monitors, and activity questionnaires. REE was measured with indirect calorimetry. We evaluated energy intake (EI) with a patient's 3-day food diary. Data was assessed employing SPSS, Spearman correlation coefficients, and Bland and Altman plots. There was no difference in DEE between the WL and WS groups measured with DLW. There were no differences in REE and EI between groups. DEE (r = 0.548, P < 0.05) and PAEE (r = 0.563, P < 0.01) are related with caloric intake. The WL group had higher PA than the WS group (P < 0.042) only when measured with wrist activity monitors. Results suggest that WL in PD patients cannot be fully explained by an increase in DEE. Large longitudinal studies to examine multiple relationships between variables might provide us with a better understanding of WL among PD patients.