Background: Premature infants fed pasteurized mature donor human milk (DHM) from milk banks have been documented to be at higher risk of developing hyponatremia. Premature infants with a history of hyponatremia have been noted as at a higher risk of suboptimal growth and neurodevelopmental outcomes. In this study, we compared infants with documented hyponatremia vs matched controls to identify the clinical risk factors of low serum sodium (Na) levels and discharge growth parameters. Materials and methods: In this retrospective study, preterm infants with hyponatremia (plasma Na < 135 mEq/L) were compared with a control group of matched gestational age. Demographics, details of the clinical course during their hospital stay, and growth parameters (weight, head circumference, and length) at discharge were recorded. Results: Sixty infants with hyponatremia, including 32 who received supplemental Na and 28 who did not, were compared with 29 controls with normal Na levels. Hyponatremic infants were more often male (70 vs 44.8%), Caucasian (81.4 vs 62.1%), received assisted ventilation (21.7% vs none), received more mother's own milk (49.2 vs 17.9%), had a later Na nadir (14 vs 5 days), and had a longer length of hospital stay (56 vs 43 days). After controlling for length of stay, infants who received supplemental Na did not differ from matched controls in Z-scores for weight, length, or head circumference. Conclusion: Contrary to our assumptions, most infants with hyponatremia had received more MOM, not DHM. Na chloride supplementation did not improve the growth parameters at discharge. Clinical significance: Human milk feedings may not always provide recommended Na intake in preterm infants even after fortification. In our small cohort, NaCl supplementation did not always correct serum Na levels or correct the growth parameters. Current protocols for the addition of NaCl to human milk do not consistently enhance serum Na levels and growth; further studies are needed.