The relation between plasma concentration of theophylline and number of apnea preterm infants was studied in six patients. The apnea frequency was monitored after cessation of theophylline treatment and plotted against the log plasma concentration of theophylline. It was calculated that at an average plasma concentration of 40 mumol/l (7.25 micrograms/ml) the preterm infants had two or less apnea per 12 h. On the basis of our results we suggest that 40 mumol/l is the minimum plasma concentration of theophylline for optimal effect on apnea in preterm infants.