The single- and multiple-dose absorption characteristics of a new sustained-release theophylline preparation, which has been formulated for once per day dosing in adults, were investigated in children aged 8 to 14 years. Four single doses were studied, each dose separated by 1 week. During steady state the preparation was given once daily in the morning for 1 week, and serum theophylline concentration was determined through two dosing intervals (48 hours). The product showed excellent sustained-release characteristics and consistent absorption profiles, which were not affected to any clinically important extent by the intake of various meals. After single doses, only 77% to 91% of the product was absorbed during the first 28 hours after dosing. However, bioavailability was complete both after single doses and during steady state. Eight of 14 children had steady-state fluctuations in serum theophylline levels of less than 90% when given doses once daily. Steady-state day-to-day variations in serum theophylline profiles were small in all patients except one, in whom differences up to 33 mumol/L (6 micrograms/mL) were seen (8 hours after dosing). We conclude that this formulation is completely absorbed at a sufficiently slow and consistent rate to permit acceptable fluctuations in absorption with once daily dosing for many, but not all, patients. However, it should not be used in very young children until bioavailability has been studied in this age group.