Theophylline has been administered to 76 premature infants with apnea in two therapeutic indications: curative and preventive treatment. Plasmatic theophylline and caffeine concentrations have been performed by gas chromatography for ten days for the determination of the therapeutic range. The maximum effective concentration has been defined from the data observed in 33 subjects presenting with symptoms considered as signs of toxicity. The minimum effective concentration has been determined from the data observed during the failure of treatment (n = 4) and at the reappearance of apnea at the end of treatment (n = 10). The therapeutic concentrations of theophylline has be found on average to be 3 to 8 mg/l and for the theophylline and caffeine together from 5 to 10 mg/l. The sum theophylline and caffeine data is better for therapeutic monitoring than the determination of theophylline alone.