The effect of manual chest percussion was studied in 10 patients recovering from an acute exacerbation of chronic bronchitis. All patients were studied on 2 consecutive days. Physiotherapy consisting of postural drainage and instructed coughing was given on both days. Chest percussion was included in a randomized way on either day. Percussion was associated with a small but statistically significant increase in airflow obstruction, measured as a reduction in FEV1 after the treatment. There were no differences in the deposition or clearance of inhaled radiolabelled particles between the 2 forms of treatment. Changes in oxygen saturation were small and insignificant during both forms of treatment. The study indicates that manual chest percussion is of little value as an adjunct to postural drainage and instructed coughing in the treatment of patients with chronic bronchitis.