Seasonal differences may influence levels of physical activity and outcomes of a pulmonary rehabilitation (PR) program. This study examined the effect of seasonal variations upon baseline measures and outcomes of physical activity, exercise performance, and health status in PR. Stable patients (N = 95) with chronic obstructive pulmonary disease (COPD) were studied and placed into the seasonal group corresponding to when PR commenced: spring (n = 18), summer (n = 23), autumn (n = 34), winter (n = 20). Physical activity was measured by a uniaxial accelerometer, exercise performance by the Incremental Shuttle Walking Test (ISWT), and health status by the Chronic Respiratory Disease Questionnaire-Self Reported. Patients who commenced PR in the winter had lower levels of physical activity at baseline (P = .02) and attained the largest improvements when compared with any other seasonal group (P = .04). No seasonal variation was detected in ISWT scores or health status either prior to or upon completion of PR. The results of this study suggest that levels of daily activity may be vulnerable to seasonal variations and this should be considered when examining physical activity levels in patients with COPD.