Intravenous aminophylline was administered to ten patients with cystic fibrosis (CF) to determine if the medication would improve pulmonary function and to study theophylline pharmacokinetics. Intravenous normal saline was given on another day as a control. Thoracic gas volume and airway resistance, measured in a volume displacement body plethysmograph, and maximal expiratory flow-volume curves were performed before and after each infusion. No significant improvement was noted in pulmonary function after normal saline infusion. Following aminophylline infusion. Following aminophylline infusion, significant improvement in thoracic gas volume, residual volume, specific airway conductance, and maximal expiratory flow at 60% of total lung capacity was noted. The pharmacokinetic analysis revealed a mean half-life of 4.7 hours, a total clearance of 91 mL/hr/kg, and a volume of distribution of 574 mL/kg. Intravenous aminophylline can acutely decrease airway obstruction in children with CF.