Over a one-year-period 23 children with chronic asthma were given an oral sustained-release theophylline preparation in twice-daily doses. Constant therapeutic theophylline levels were maintained in 18 children for the whole period, while in 5 subjects the values were in almost all instances below the therapeutic range. In both groups, however, a good clinical effect from the preparation was observed. Only two children reported nausea and mild abdominal pains, whereas other two children had serum theophylline levels above 20 micrograms/ml with no side effects. It is concluded that oral sustained-release theophylline preparation is effective in providing therapeutic drug concentrations on a 12 hr dose schedule, thus avoiding the risk of lower compliance observed in children given normal theophylline tablets.