Wearable sensors have been developed and used in nondisabled adults. Little is known about their accuracy in patients with chronic disease. We tested the ActiHealth accelerometer, which measures step counts, in subjects with chronic obstructive pulmonary disease (COPD). We determined the intra- and interdevice coefficients of variation (CVs). We assessed the accuracy of the device in 15 nondisabled males and 46 subjects with COPD. Accuracy was defined as percent step capture, (device step count divided by manual step count) times 100. Predictors of percent step capture were identified using linear regression methods. The accelerometer has an intradevice CV ranging from 0.008 to 0.025 and an interdevice CV of 0.64. In nondisabled males, median percent step capture was 96% (interquartile range 81%-98%). In subjects with COPD, median percent step capture was 86% (interquartile range 72%-96%). Usual walking speed was the most important predictor of percent step capture (p = 0.004). The ActiHealth accelerometer has acceptable intra- and interdevice CVs. It is highly accurate in nondisabled subjects. The accuracy declines in subjects with COPD based on walking speed. Prior to using the ActiHealth accelerometer, researchers and clinicians should assess walking speed and percent step capture in each subject.