A cluster of cases of necrotizing enterocolitis among extremely preterm infants (<33 weeks’ gestation) hospitalized in the intensive care and/or neonatal intensive care units at the Caen University Hospital was reported in April 2015. Initial investigations (bacteriological, virological, environmental, and food samples, etc.) failed to identify a single infectious cause. A complementary epidemiological and etiological study was initiated to investigate, in particular, dietary risk factors. A retrospective cohort study was conducted, including all infants under 33 weeks’ gestational age and weighing less than 1,500 g who were hospitalized in the neonatal intensive care unit and/or neonatal resuscitation unit between January 14 and April 16, 2015. A case was defined as definite, probable, or suspected according to the modified Bell classification. In univariate analysis, qualitative and quantitative variables were compared using Fisher’s exact test and the Kruskal-Wallis test, respectively. The results of the univariate analysis were adjusted for gestational age using a multiple regression model. Nine cases and 24 non-cases were identified during the study period. Among the 9 cases, 5 were definite, 3 were possible, and 1 was suspected (excluded from the analysis). Among the 24 non-cases, 4 were excluded from the analyses because they had a life-threatening neonatal condition that caused their early death (< 3 weeks). The median gestational age was 26.5 weeks in cases and 31 weeks in non-cases (p=0.0179). The median birth weight was 885 g in cases and 1,300 g in non-cases (p=0.0034). The median 5-minute Apgar score was slightly higher in cases (p=0.008), and a history of gestational diabetes was more common among the mothers of cases (p=0.011). The median duration of parenteral nutrition was longer in cases (30 days vs. 19 days in non-cases, p=0.038). The distribution of types of milk received and other dietary factors studied among cases and non-cases did not differ over the exposure period. Among the other factors studied (invasive procedures, imaging studies, physical therapy, fundus examination), none were associated with a higher risk of ECN. The cohort study identified gestational age as the primary risk factor.