The aim of this study was to estimate, in patients with chronic obstructive pulmonary disease (COPD), the maximal strength of the expiratory muscles, its correlation with exercise performance and the effects of a specific physiotherapy. In 38 COPD men, aged 54 +/- 7 years, pulmonary function data, maximal alveolar pressure (Palv, max) developed during forced vital capacity, were measured using a whole-body plethysmograph and the maximal tolerated power (MTP), i.e. the highest power maintained for at least 3 min, was determined by a progressive test on a treadmill. Airway obstruction was severe (FEV1/FVC: 54 +/- 10%), Palv, max was lower than normal (74 +/- 36 vs. 130 +/- 48 hPa in 20 healthy men of the same age; p less than 0.01) and increased with airway resistance values (Raw); mean MTP was low: 115 +/- 30 W and individual values were inversely related to Raw values. Then, two subgroups of 14 patients were chosen at random. One subgroup received an abdominal muscle physiotherapy during 3 weeks. The other subgroup only received usual medical treatment. No modification in any parameter was found in the second subgroup. Specific physiotherapy of abdominal muscles improves significantly both Palv, max (118 +/- 45 hPa) and MTP (171 +/- 38 W; p less than 0.01), without any variation in other respiratory function parameters. We conclude that abdominal muscle weakness is common in COPD patients and can participate in the limitation in exercise performance. Specific physiotherapy increases abdominal muscle strength and seems to improve exercise tolerance by a still unexplained mechanism.