Problem Statement: Despite great efforts to prevent necrotizing enterocolitis (NEC) the incidence may in fact be increasing, and changes in the patient population over time seem to lead to changes in clinical presentation and risk factor spectrum as well. The presence of bacteria is an important prerequisite in the pathogenesis, but, rather than being caused by specific pathogens, inflammation and bacterial invasion are thought to be mediated through erroneous interaction between microbiota and innate immunity during colonization of the gut. There are, however, reports of episodic outbreaks of NEC, seasonal variation in incident rates, and clustering, suggesting a role for transmissible infectious agents or other environmental factors around the pregnant mother or newborn infant. In order to investigate evidence for such factors we have analyzed the occurrence of space-time clusters in Sweden over 23 years. Methods: A national register-based cohort of all children born between 1987 and 2009 in Sweden, diagnosed with NEC, was identified. The Knox test and Kulldorff’s scan method were used to analyze signs of space-time clusters at two geographical levels; the mother’s residential address and the delivery hospital. Time windows of seven, 14 and 21 days were used for closeness in time. Results: The Knox test showed clustering on hospital level in all studied temporal windows; seven days (p=0.022) 14 days (p=0.011) and 21 days (p=0.006), and Kulldorff’s scan method found seven significant clusters. On residential level, there was no indication of space-time interaction. When comparing two time periods, significant clustering on hospital level was found during 1987-1997, but not during 1998-2009. Conclusion: Space-time clustering was found on hospital level, but not on community level, suggesting a contagious environmental effect at and after delivery but not in the materno-fetal environment outside the hospital before birth. The decrease in clustering over time suggests that improved routines in neonatal care have minimized the risk of NEC precipitating contagions spreading between patients in the neonatal intensive care unit. The importance of such routines should not be forgotten while our efforts to bring down NEC incidence are directed towards other challenges.