Objectives: To assess the changes in physical activity in subjects with chronic obstructive pulmonary disease over 6 months after pulmonary rehabilitation. Design: Prospective, observational study. Activity was measured over 2-day periods at the end of rehabilitation, and repeated every 6 weeks for 6 months using the ActivPAL uni-axial accelerometer. These results were compared with the shuttle walking test (SWT) and the St. George's Respiratory Disease Questionnaire (SGRDQ). Setting: UK community hospital. Participants: Adults completing a community rehabilitation programme. Main outcome measure: Time spent standing and mobilising ('uptime'). Results: Of 34 subjects recruited, 28 completed the 6-month study period (mean age 69 years, mean forced expiratory volume in 1 second 1.3 l). Participants wore the monitor for 13.8 to 14.2 hours/day. At baseline (post-rehabilitation), participants spent 1.7 [standard deviation (SD) 1.3] hours/day walking and 3.5 (SD 2.6) hours/day standing. Taking the group as a whole, mean uptime decreased marginally by 13.6 minutes after 24 weeks compared with baseline, with significant individual variability. In all but one subject who showed decreased activity, this was apparent after 6 weeks. There were no significant changes in the mean SWT or SGRDQ. Significant associations between total uptime and the SWT were found, but coefficients were weak. It was not possible to predict individual responses from baseline data. Conclusion: The accelerometer provides useful supplementary data in patients completing rehabilitation programmes, and the results reveal wide variation. The weak associations between activity data and the SWT suggest that monitors provide additional information. More work is required to determine the factors associated with early deterioration in activity in order to design appropriate interventions.