Introduction: In this study, we aimed to define the incidence and time to detection of late onset hyponatremia (LOH) as well as factors affecting its development in preterm newborns. We also aimed to determine the daily sodium requirement of these patients. Methods: We studied a total of 145 very low birth weight infants with a full or nearly full enteral diet and followed them up until discharge. We recorded demographic and clinic characteristics. We measured serum sodium (SNa) levels at least once a week after the second week. We compared infants with LOH with other infants to analyze possible risk factors. Results: Twenty-nine (20%) infants developed LOH in an average of 23.4 +/- 7.8 days. The mean SNa level of these infants was 124.6 +/- 5.6 mmol/L. Logistic regression analysis showed that a birth weight of less than 1000 g, preterm early membrane rupture, and nutrition with fortified human milk alone were risk factors for LOH. The mean daily amount of sodium added to the nutrition of hyponatremic preterm infants was 3.6 +/- 2.1 mmol/L. Subgroup analysis showed that the incidence of LOH was two times higher (39.2%) in infants with a birth weight of less than 1000 g. Conclusion: We observed the development of LOH within three to four weeks in nearly half of preterm infants fed with fortified human milk, especially those with a birth weight of less than 1000 g. We believe that the sodium content of currently used human milk fortifiers should be increased.