The bronchodilating action of theophylline in COPD has been examined, with emphasis on its combined use with inhaled beta 2 agonists. The suggestion is made that failure to recognize the nonlinearity of the dose-response curves for bronchodilators has resulted in underestimating their combined action. Recent studies suggest that systemic theophylline has somewhat different actions on the airways in COPD than inhaled beta agonists, and that more bronchodilation may be possible when the two are used together than large doses of either one. By analogy, with asthma the suggestion is also made that the addition of theophylline is also likely to provide a more constant bronchodilation, reducing peak-trough variations in flow. The most complete clinical comparison to date suggests that, in currently sanctioned doses, a regimen containing both theophylline and an inhaled beta 2 agonist provides significantly greater bronchodilation than either drug alone, with fewer patient withdrawals. Further carefully designed studies are needed to resolve this issue, and particularly, to identify those patients who will derive the greatest benefit from a combined regimen.