We studied the occurrence of necrotizing enterocolitis in 2681 very low birth weight infants during an 18-month period to characterize the biodemographic and clinical correlates. Proven necrotizing enterocolitis (Bell stage II and beyond) occurred in 10.1% of study infants; necrotizing enterocolitis was suspected in 17.2% of study infants. Positivity of blood cultures was related to necrotizing enterocolitis staging. The mortality rate increased only for stage III necrotizing enterocolitis (54% died). Logistic regression identified medical center of birth, race, gender, birth weight, maternal hemorrhage, duration of ruptured membranes, and cesarean section as significant risk factors. For one center the odds ratio was 3.7, whereas for another center it was only 0.3. For black boys, the odds ratio was 2.3 relative to nonblack boys; for girls, race did not affect prevalence of necrotizing enterocolitis. Age at onset was related to birth weight and gestational age. Intercenter differences in necrotizing enterocolitis prevalence were related to time required to regain birth weight and other indicators of fluid management. Gram-positive organisms predominated in positive blood cultures for stage I and II necrotizing enterocolitis; enteric bacteria were isolated more frequently in infants with stage III disease. We conclude that necrotizing enterocolitis prevalence varies greatly among centers; this may be related to early clinical practices of neonatal care.