We studied the effect of cimetidine and theophylline on the metabolism of each drug in 19 elderly patients with stable chronic obstructive pulmonary disease. Each patient received either regular release or sustained release oxtriphylline. Cimetidine was given to some patients in both groups after steady state for theophylline was achieved. Multiple serum theophylline and cimetidine measurements were performed. Cimetidine decreased theophylline clearance by an average of 37.8% (26.2 to 46.7) for regular release and 33.4% (22.2 to 45.0) for sustained release oxtriphylline. The apparent volume of distribution was not changed; however, the half-life for theophylline was increased. As there is a linear relationship between the theophylline clearance before and after the addition of cimetidine and a linear relationship between trough and peak values before and after cimetidine, these 2 relationships may be used to predict clearance values and serum levels after cimetidine treatment when the initial values are known.