Background: Dysnatremia after surgery for congenital heart disease (CHD) is well known and has been associated with prolonged pediatric intensive care unit length of stay (PICU-LOS). Fluctuations in plasma sodium levels occur perioperatively. The primary aim of the study was to evaluate the occurrence of dysnatremia during the first 48 h after surgery and whether it was associated with PICU-LOS. The secondary aim was to evaluate if the degree of sodium fluctuations was associated with PICU-LOS. Methods: A retrospective observational, single-center study including infants undergoing surgery for CHD. The highest and lowest plasma sodium value was registered for the prespecified time periods. PICU-LOS was analyzed in relation to the occurrence of dysnatremia and the degree of plasma sodium fluctuations. The occurrence of dysnatremia was evaluated in relation to surgical procedure and fluid administration. Results: Two hundred and thirty infants who underwent 249 surgical procedures were included. Dysnatremia developed in more than 60% within 48 h after surgery. Infants with normonatremia had a 40%–50% shorter PICU-LOS among children in RACHS-1 category 3–6, compared with infants developing either hypo- or hyper-/hyponatremia within 48 h after surgery (p =.006). Infants who had a decline of plasma sodium >11 mmol/L had almost double the PICU-LOS compared to those with a decline of <8 mmol/L. Conclusion: Dysnatremias were common after surgery for CHD and associated with prolonged PICU-LOS. The degree of decline in plasma sodium was significantly associated with PICU-LOS. Fluid administration both in terms of volume and components (blood products and crystalloids) as well as diuresis were related to the occurrence of dysnatremias.