Long-term treatment of chronic bronchitis with chest physiotherapy with or without positive expiratory pressure (PEP) by mask was studied in 43 patients randomly allocated to PEP treatment (PEP group, 20 patients) and conventional chest physiotherapy (control group, 23 patients). After instruction, the treatments were self-administered twice daily for 12 months (34 patients) and 5 months (9 patients). Twice weekly, patients filled in a diary concerning symptoms. The PEP group had significantly less cough and less mucus production. The number of acute exacerbations were calculated from the diaries and were lower in the PEP group compared to the control group, and 85 percent of the patients in the PEP group were free from acute exacerbations versus 48 percent in the control group. The PEP group also used less antibiotics and mucolytics. The PEP group had a small increase in FEV1 of mean 62 ml compared to a small decrease of 43 ml for the control group. Treatment with a simple PEP device can reduce morbidity in patients with chronic bronchitis and may preserve lung function from a more rapid decline.