Twenty-six infants had necrotizing enterocolitis (NEC) in the neonatal unit of the Long Island Jewish-Hillside Medical Center, New Hyde Park, NY, between 1964 and 1976; 25 of these cases occurred between 1973 and 1976. The relationship of feeding practices to this clustering of NEC cases was investigated in two ways: (1) A 10% sample of all admissions, 1964 to 1976, was studied. A striking correlation was found between the yearly incidence of NEC and the percentage of infants in that year who had received large increases in daily feeding volume. The highest mean daily feeding volume occurred in 1974 and 1975, the two years of peak NEC incidence. (2) The feeding records of the 26 NEC cases were studied. Thirteen NEC patients had had large increases in feeding volume within two days of NEC onset. Seven others received greater than 150 mL of formula per kilogram per day prior to NEC onset. Rapid increase in feeding volume and the use of large volumes are suggested as important etiologic factors of NEC.