The traditional role of theophylline as a bronchodilator has been expanded by recent findings that suggest this drug has more than smooth muscle relaxant properties. Several investigators indicate that theophylline has an inotropic effect on respiratory muscle, causing enhanced muscular contraction and prevention of muscle fatigue. In animal studies, the drug enhanced respiratory muscle contraction by 15-20%, with levels in the upper end of the therapeutic range (15-20 mg/L). Results of studies in healthy volunteers and patients with lung disease, however, are conflicting. Five clinical trials demonstrated increased diaphragmatic contractility, whereas seven trials showed no effect, with five referring to the diaphragm and the remaining two to the sternomastoid muscle. Disparity in outcomes may be attributed to differences in patient populations, study designs, and techniques used to determine diaphragmatic contractility. Few long-term trials exist that document significant clinical benefit. Theophylline may prove to be of value in selected populations, such as adults with hypercapnic obstructive lung disease.