The utility of therapeutic drug monitoring was studied in children with apnea. Plasma levels of theophylline were monitored in 106 patients. Of these 106 patients, 19% had subtherapeutic levels (group 1), 60% had therapeutic levels (group 2), and 21% had toxic levels (group 3). After we adjusted dosages and instituted monitoring, theophylline levels for group 1 rose to 8.12 mcg/ml on average and levels in group 3 were 8.3 mcg/ml. Another group of children who were not monitored spent from 14 to 26 days in the hospital; the monitored patients spent from 9 to 19 days. We concluded that monitoring of theophylline levels is supportive for newborns with apnea.