During a 2-month period, 13 infants in this neonatal intensive care unit developed necrotizing enterocolitis, increasing the prevalence in inborns from 5.2 to 20.5/1,000 live births. Fifty-seven perinatal and neonatal factors, many of which have previously been associated with necrotizing enterocolitis, were compared between the infants with necrotizing enterocolitis and 17 unaffected inborn control infants admitted concurrently. Clostridium difficile cytotoxin was detected in the stools of 12 affected infants (92.3%) in comparison with two control infants (11.8%) (P less than .001), and the organism was isolated in eight affected neonates (61.5%) compared to none of the control infants (P less than .001). The outbreak was terminated upon institution of oral vancomycin therapy in cases and infant contacts, and strict antiinfective measures in the neonatal intensive care unit. This indicates an etiologic role of C difficile in the outbreak. Oral vancomycin in the management of necrotizing enterocolitis was assessed by therapeutic response, drug levels, and occurrence of side effects. Oral vancomycin therapy is indicated in necrotizing enterocolitis outbreaks in units where C difficile is endemic.