Over a 3-month period 22 asthmatic children were treated with a sustained-release theophylline preparation (Nuelin Retard, Riker) in twice-daily doses. During the first 2 weeks of treatment slight side-effects were observed in 14 children. Over the remaining 10-week period only four children experienced minor-side-effects. Serum theophylline levels 4 hours after tablet administration were measured three times and serum levels just before tablet administration were measured once during the observation period. The average difference between the serum levels of theophylline just before tablet administration and the corresponding serum value 4 hours afterwards was 4.2 microgram/ml. The serum values 4 hours after tablet administration showed an insignificant fall throughout the period and patient compliance was assessed as good. When the average requirements of theophylline in mg/kg found by Wyatt et al. (15) (Table 2) were used as initial doses, few dosage adjustments were necessary and no serum values were greater than 26.2 microgram/ml, but in order to avoid serum levels above 20 microgram/ml initial doses should be decreased about 10% and final adjustment made with the aid of serum theophylline measurement under steady state conditions. In children, Nuelin Retard given in two daily doses is capable of maintaining a constant therapeutic serum concentration for the whole 24-hour period.