Improved neonatal management has resulted in an enlarging population of extremely low birth weight (ELBW) infants. These infants have a high incidence of necrotizing enterocolitis (NEC) and a high mortality rate. The authors compared two groups of NEC patients: ELBW infants (< 1,000 g and/or < or = 28 weeks' gestation) and "standard" premature infants (29 to 36 weeks' gestation). NEC was classified according to the extent of bowel involvement: (1) focal, (2) diffuse, or (3) pan involvement (pan necrosis). Clinical laboratory, radiological, pathological, and bacteriologic findings, management, and mortality were analyzed. There were no significant differences between the groups with respect to gender, race, delivery mode, or incidence of prenatal or perinatal problems. The most common presenting signs in both groups were abdominal distension, vomiting, and feeding intolerance. The onset of signs and the time of first feedings were significantly later in the ELBW group. Pneumatosis was the most frequent initial radiological finding (60% of the ELBW group, 75% of the premature group). Portal vein air (PVA) was present in 29% of the ELBW and premature infants. Seventy-one percent of ELBW infants with PVA had pan involvement, versus 40% of premature infants (P < .05). There were significant differences in the peritoneal cultures between the groups. The premature group had significantly more Escherichia coli (54% v 23%). The ELBW group had a wider variety of microorganisms (eg, Clostridium sp, Pseudomonas sp, and yeast). Survival was significantly higher for the premature group (84% v 55%). The mortality rate was 93% when pan involvement was present in the ELBW group.(ABSTRACT TRUNCATED AT 250 WORDS)