Introduction: Newborns with gastroschisis require appropriate fluid resuscitation but are also at risk for hyponatremia that may lead to adverse outcomes. The etiology of hyponatremia in gastroschisis has not been defined. Methods: Over a 24-month period, all newborns with gastroschisis in a free-standing pediatric hospital had sodium levels measured from serum, urine, gastric output, and the bowel bag around the eviscerated contents for the first 48 h of life. Total fluid intake and output were measured. Maintenance fluids were standardized at 120 mL/kg/day. Hyponatremia was defined as a serum sodium <132 mEq/L. A logistic regression model was created to determine independent predictors of hyponatremia. Results: 28 infants were studied, and 14 patients underwent primary closure. While serum sodium was normal in all patients at birth, 9 (32%) infants developed hyponatremia at a median of 17.4 h of life. On univariate analysis, hyponatremic babies had a greater net positive fluid balance (74.9 vs 114.7 mL/kg, p = 0.001) primarily due to a decrease in total fluid output (p = 0.05). On multivariable regression, a 10 mL/kg increase in overall fluid balance was associated with an increased risk of developing hyponatremia (OR 1.84 [1.23, 3.45], p = 0.016). No differences in the sodium content of urine, gastric, or bowel bag fluid were observed, and sodium balance was equivalent between cohorts. Discussion: Hyponatremia in babies with gastroschisis in the early postnatal period was associated with positive fluid balance and decreased fluid output. Prospective studies to determine the appropriate fluid resuscitation strategy in this population are warranted.