Discriminating necrotising enterocolitis (NEC) and focal intestinal perforation (FIP) is important for clinical trials, observational cohorts, quality improvement and aetiological understanding. Literature suggests that timing and key features diagnose and discriminate, and that NEC subclassifications exist. We used a detailed 10-year cohort of NEC and FIP cases in preterm infants born <32 weeks’ gestation from a single centre to explore antecedent factors, presentation and potential NEC subclassifications. 785 infants had 144 episodes of NEC and 38 of FIP. FIP presented earlier than NEC, but ranges overlapped, and 30% of NEC presented before day 14. Antecedent events (other than feed volumes) and outcomes did not differ between NEC and FIP. Currently used diagnostic/discriminatory features performed poorly, and subclassification identified few cases except transfusion-associated NEC. Contrary to existing literature, postnatal age at NEC presentation was not dependent on gestational age. Detailed review rather than simple definitions are required to separate NEC from FIP.