This study examined the validity of the StepWatch activity monitor and the optimal number of monitoring days in patients with chronic obstructive pulmonary disease (COPD). StepWatch activity data were obtained from databases of two studies: one of patients with COPD who participated in a trial of two exercise persistence interventions (n = 17) and a second, cross-sectional study of activity in chronically healthy older adults (n = 60). Independent t tests, Pearson's correlations, and Bland-Altman plots were used to examine the activity data (total steps, activity intensity, and distribution of activity). Results indicated that patients with COPD registered 50% fewer total steps and had lower levels of walking intensity compared with chronically healthy older adults. Activity parameters had moderate-to-high correlations with disease severity, peak cycle workload, and physical functioning. More monitoring days were associated with greater measurement precision. This study provides preliminary evidence to support the validity of the StepWatch device for measuring free-living ambulatory physical activity in COPD.