The purpose of this study was to determine the effect of respiratory muscle training on muscle strength and endurance, exercise capacity, and functional status in patients with chronic airflow limitation. Computerized bibliographic data bases (MEDLINE AND SCISEARCH) were searched for published clinical trails, and an independent review of 73 articles by two of the investigators identified 17 relevant randomized trials for inclusion. Study quality was assessed and descriptive information concerning the study populations, interventions, and outcome measurements was extracted. We combined effect sizes across studies (the difference between treatment and control groups divided by the pooled standard deviation of the outcome measure). Across all studies, the effect sizes and associated p-values were as follows: maximal inspiratory pressure 0.12, p = 0.38; maximal voluntary ventilation 0.43, p = 0.02; respiratory muscle endurance 0.21, p = 0.14; laboratory exercise capacity -0.01, p = 0.43; functional exercise capacity 0.20, p = 0.15; functional status 0.06, p = 0.72. Secondary analyses suggested that endurance and function may be improved if resistance training with control of breathing pattern is undertaken. Overall, there is little evidence of clinically important benefit of respiratory muscle training in patients with with chronic airflow limitation. The possibility that benefit may result if resistance training is conducted in a fashion that ensures generation of adequate mouth pressures may be worthy of further study.