Sustained release theophylline products can improve compliance and symptom control in children with asthma. This study examines theophylline serum concentration monitoring in pediatric patients. Fifteen children with documented asthma were randomized to receive either Slo-bid Gyrocaps® or Theo-dur Sprinkle® for 1 month, and then crossed over to the other product. On the last day of each study period, theophylline serum concentrations were obtained prior to the morning dose and 4 hours later. In two patients receiving Theo-dur Sprinkle® and six with Slo-bid Gyrocaps®, the 4-hour serum concentration was lower than the pre-dose concentration. The change between the pre-dose and post-dose serum concentrations for Theo-dur Sprinkle® ranged from a decrease of 2.8 mg/L to an increase of 4.9 mg/L, and, for Slo-bid Gyrocaps®, from a decrease of 4.6 mg/L to an increase of 10.5 mg/L. The inconsistent theophylline absorption with each product makes dosage adjustment difficult.