Failure of adequate gastric emptying frequently prevents successful, early enteral nutrition in the pre-term infant. The effect on gastric emptying of adding breast milk fortifier is unknown, but clinical experience suggests that it is less well tolerated by some infants. We therefore compared gastric emptying rates of breast milk and fortified breast milk within pre-term infants, using a previously described ultrasonic technique. Eleven infants were studied on 22 occasions. Median (range) gestation of the group was 28 weeks (25-31) with birth weight 1090 g (714-1360). The human milk fortifier FM-85 (Nestlé, Vevey, Switzerland) was used in all infants. Half-emptying time for unfortified breast milk was less than half that for fortified breast milk. Mean (+/- SEM) half emptying times were 21 min (+/- 3.6) and 48 min (+/- 4.0), respectively. Breast milk emptied faster than fortified breast milk in 10 out of 11 patients. These data demonstrate that the addition of human milk fortifier can significantly slow gastric emptying. This has important implications for the management of infants who have feed intolerance.