In two groups of patients, 15 with asthma and 15 with chronic bronchitis, the bronchodilator effects of ipratropium bromide, of fenoterol plus theophylline, and of the combination of the three drugs, were compared using a double-blind, single-dose, placebo-controlled format. Ipratropium bromide caused rapid bronchodilatation which was not significantly different in asthmatic patients and patients with bronchitis (delta FEV1 = .29 L in one hour in asthmatic patients, .18 L in patients with bronchitis). In contrast, fenoterol plus theophylline induced a considerably greater effect in asthmatic patients (delta FEV1 = .41 L in one hour) than in those with bronchitis (delta FEV1 = .07 in one hour). The use of the three drugs in combination compared with ipratropium bromide alone, or fenoterol plus theophylline alone, resulted in a significant additional bronchodilatation in asthmatic patients. In the patients with bronchitis, the triple combination was clearly superior to fenoterol plus theophylline. A similar trend was present in comparing the triple combination to ipratropium bromide, but the difference did not reach statistical significance. There was no evidence of synergism when ipratropium bromide was combined with fenoterol plus theophylline in that the total bronchodilator effect was approximately additive. Asthmatic patients and the physician were able to distinguish the triple combination from placebo. No such ability was demonstrated with respect to those with bronchitis. All three drugs were well tolerated. Side effects were mostly mild, and none was related to the use of ipratropium.