To develop appropriate theophylline dosage recommendations for infants < or = 1 year of age, we evaluated the Nassif, Hendeles, and Hatzopoulos dosing equations in 75 infants who were receiving theophylline intravenously by continuous infusion. Postnatal age ranged from 1 to 52 weeks and postconceptional age from 33 to 93.8 weeks. Using each patient's measured theophylline clearance at steady state and the dose recommended by each of the three equations, we predicted a steady-state serum theophylline concentration for the three equations. The recommended theophylline dose and resultant serum concentrations differed significantly for the three equations. The Nassif, Hendeles, and Hatzopoulos equations resulted in 78.7%, 97.3%, and 93.8%, respectively, of serum theophylline concentrations between 5 and 15 mg/L. The Hendeles and Hatzopoulos equations tended to produce concentrations between 5 and 10 mg/L in the majority of infants; the Nassif equation generally resulted in values between 10 and 20 mg/L. Four percent of the calculated serum concentrations with the Nassif equation were > 20 mg/L. When a desired concentration of 10 mg/L was used in the Hatzopoulos equation, predictions of theophylline concentrations were consistently inflated. We conclude that the Hendeles equation ((0.008 x Postnatal age in weeks) + 0.21 = mg/kg per hour or mg/kg daily = (0.2 x Postnatal age in weeks) + 5) is preferred when intravenous theophylline therapy for apnea and bradycardia or for asthma is initiated. Regardless of the equation used to estimate an initial theophylline dose in infants < or = 1 year of age, serum theophylline concentrations should be monitored within 6 to 12 hours after the start of therapy.