Sustained-release theophylline formulations should be most useful in young children who have rapid clearance and long sleep intervals. Somophyllin-12 is a recently introduced, newly designed, bead-filled capsule. We tested its ability to provide adequate serum concentrations at steady state in 16 children ages 0.9 to 5.1 yr. On a 12 hourly dosing schedule, mean dose was 25.9 mg/kg per day, and mean fluctuation was 138%. Seven children (43.8%) had fluctuations less than 100%, whereas four children (25%) had fluctuations more than 200%. Six patients with excessive fluctuation (greater than 100%) were restudied on an eight hourly dosing schedule. Mean dose was 28.7 mg/kg per day, and fluctuation was 59%. Because of the excessive fluctuation (greater than 100%) experienced by half these young children, therapy with this product should be initiated with an eight hourly dosing schedule. However, when dose and dosing interval are subsequently individualized, many young children can be switched to twice daily dosing with its improved compliance and convenience.