BACKGROUND: A construct calculated as the sum of items 13-15, 29, 30 of the Unified Parkinson's Disease Rating Scale (UPDRS) has been used as an "Ambulatory Capacity Measure" (ACM) in Parkinson disease (PD). Its construct validity has never been examined. A similar construct, consisting of the mean value of the same UPDRS items has been used under the acronym PIGD as a measure of postural instability and gait disorder in PD. OBJECTIVE: To examine the construct validity of the ACM and PIGD in PD. METHODS: We analyzed data in an existing database of 340 PD patients, Hoehn and Yahr stages (HYS) 1-5 who participated in a study of falls. Number of falls (NOF) was recorded over 4 weeks, and UPDRS (mental, ADL, and motor subscales), HYS, Activities Based Confidence Scale (ABC), Freezing of Gait Questionnaire (FOG), Five Times Sit-to-Stand (FTSS), Timed Up-and Go (TUG), Gait Velocity (GV), and Berg Balance Scale (BBS) evaluations were performed. Internal consistency was assessed by Cronbach's alpha. Construct validity was assessed through correlations of the ACM and PIGD to these measures and to their summed-ranks. A coefficient of determination was calculated through linear regression. RESULTS: Mean age was 71.4, mean age at diagnosis 61.4 years; 46% were women; mean UPDRS subscale scores were: Mental 3.7; ADL 15.7; motor: 27.1; mean ACM was 6.51, and mean PIGD 1.30. Cronbach's alpha was 0.78 for both ACM and PIGD. Spearman correlation coefficients between the ACM/PIGD and ABC, FOG, TUG, GV and BBS were 0.69, 0.72, 0.67, 0.58, and 0.70 respectively. Correlation between the ACM/PIGD and summed-ranks of HYS, NOF, ABC, FOG, FTSS, TUG, GV and BBS was high (Spearman r = 0.823, p < 0.0001); 68% of the variability in the summed-ranks was explained by ACM/PIGD. CONCLUSION: The ACM and the PIGD are valid global measures and accurately reflect the combined effects of the various components of ambulatory capacity in PD patients with HY stages 1-4.