The incidence of necrotizing enterocolitis encountered in neonates fed only refrigerated human milk was comparable to that in infants fed milk and isotonic formula or isotonic formula alone. The infants fed human milk were significantly (P less than 0.05) smaller, less mature, had lower Apgar scores, and were fed later than the formula-fed infants. The mean age of onset and time between first feeding and onset of NEC was similar among the three groups. These data indicate that refrigerated human milk was not effective in lowering the incidence of NEC. Possible explanations for the occurrence of NEC in neonates fed human milk include: (1) the introduction of a pathogen via contaminated milk; (2) inadequate maternal antigenic stimulation by the neonatal gastrointestinal flora; and (3) adverse affects of storage on cell number and function.