This study was conducted to determine if piroxicam alters the clearance or metabolism of theophylline in healthy adults. Six male volunteers were given aminophylline 6 mg/kg iv before and after piroxicam treatment. Piroxicam 20 mg/d po was administered for seven days. Serum samples were collected before and for 24 hours after each aminophylline dose. Theophylline serum concentrations were determined using the Abbott TDx automated immunoassay system. Theophylline and its 1,3-dimethyluric acid metabolite were measured in 24-hour urine collections. Serum theophylline concentration versus time curves were plotted and pharmacokinetic parameters determined for each subject. None of the derived parameters differed as a result of piroxicam exposure. Theophylline metabolites, clearance, half-life, and volume of distribution were compared during the baseline and piroxicam phases and were found to be similar. It was concluded that piroxicam does not alter the clearance, distribution, volume, or metabolism of theophylline in healthy adult volunteers.