Six children and six adults were given a single dose of a sustained release theophylline preparation (Nuelin Retard) after an overnight fast and later after a standardized breakfast. Food significantly reduced the absorption rate of theophylline whereas the extent of absorption of theophylline was unaffected by food. Under steady state treatment the fluctuations in the serum levels of theophylline were small when the tablets were taken after meals twice a day showing that this dose regimen was sufficient. No characteristic absorption or elimination phases were seen during the steady state dose interval, and the time to peak serum theophylline level showed great variation. Serum theophylline control about 4 h after fasting intake of the tablet is recommended.