Twenty-four samples of cord blood chosen at random from premature newborns were analyzed for methylxanthines. At least one methylxanthine could be detected in 20 (83%) samples, and caffeine was present in 17 (71%). In 8 (33%) samples the serum concentration was above 15 mumol/l; above this level an effect on ventilation has been demonstrated. Of 50 samples of human milk, 48 (96%) contained at least one methylxanthine, and in 42 (84%) caffeine could be detected. In treating premature infants with caffeine or theophylline during the first days of life a possible prenatal supply has to be taken into consideration, however, a supply via human milk can be neglected.