Theophylline is a commonly used bronchodilator in the treatment of chronic obstructive airways disease (COAD) with a narrow therapeutic range of 10 to 20 micrograms/ml. Patients with COAD frequently receive concomitant antibiotic therapy for respiratory infections. This study evaluated the effect of tetracycline therapy on theophylline disposition in adults with COAD. Six males (five nonsmokers) with obstructive ventilatory defects were studied in two phases: control, after receiving sustained-release theophylline in the same dosage regimen for four days, and treatment, after receiving tetracycline 250 mg po qid for five days in addition to theophylline. During each phase, 10 blood samples were obtained over one dosing interval and analyzed for theophylline content. The following pharmacokinetic parameters were calculated: Cmin, Cmax, Css, percentage fluctuation and Cl. Differences for each value were tested as paired data with Student's two-tailed t-test. When all patients were evaluated, the only statistically significant difference was for Cmax. However, when the five nonsmokers were evaluated separately, differences were observed for Css (micrograms/ml; mean +/- SD) 9.3 +/- 3.0, control, and 10.6 +/- 3.8, treatment (p = 0.041); and for Cl [( ml/h]/kg; mean +/- SD) 49.0 +/- 11.1, control, and 43.6 +/- 10.2, treatment (p = 0.019). This study demonstrates that tetracycline may weakly inhibit theophylline clearance in nonsmoking adults with COAD.