Steady-state serum theophylline concentrations following equal doses of intravenous aminophylline and oral theophylline were compared in 30 preterm infants with gestational age of 29.2 +/- 2.9 weeks. The result showed no significant statistical difference between the mean serum concentration of theophylline (8.2 +/- 2.2 micrograms/mL vs. 8.4 +/- 1.9 micrograms/mL; p = 0.483). This indicates that a dose reduction of 20%, which is currently recommended, is not required when changing from intravenous aminophylline to oral theophylline. We conclude that in preterm infants, equal doses of intravenous aminophylline and oral theophylline maintain the same serum theophylline concentration.