Combination bronchodilator therapy for chronic obstructive pulmonary disease (COPD) is available widely throughout the world except in North America. Previous studies have yielded conflicting results regarding the advantages of combining anticholinergic therapy with sympathomimetic therapy in COPD. We report the results of a 12-week prospective, double-blind, parallel-group evaluation of the use of the following: albuterol, a beta-adrenergic agent; ipratropium, an anticholinergic agent; or a combination of the two, administered by metered-dose inhaler to patients with moderately severe stable COPD. Following baseline studies, 534 patients were given one of the three test bronchodilator preparations to be used at home four times daily in addition to oral theophylline and corticosteroids as required. The doses of the latter two drugs were kept stable. Subjects were tested on days 1, 29, 57, and 85. Analysis of 1-s forced expiratory volume (FEV1) curves on those test days indicated that the combination was superior to either single agent alone in peak effect, in the effect during the first 4 h after dosing, and in the total area under the curve of the FEV1 response. The mean peak percent increases in FEV1 over baseline on the four test days were 31 to 33 percent for the combination, 24 to 25 percent for ipratropium, and 24 to 27 percent for albuterol. The differences between the combination and its components were statistically significant on all test days. The AUC0-4 means for the combination were 21 to 44 percent greater than the ipratropium means and 30 to 46 percent greater than the albuterol means. Similar changes were noted in the forced vital capacity curves. Symptom scores did not change over time and did not differ among the treatment groups. We conclude that the combination of ipratropium and albuterol, when given by metered-dose inhaler to patients with COPD, is more effective than either of the two agents alone. The advantage of the combination is apparent primarily during the first 4 h after administration. The availability of combination therapy by metered-dose inhaler should help to improve patient compliance.