OBJECTIVES: Hypernatremia may facilitate the diffusion of bilirubin through the blood-brain barrier and increase the risk of bilirubin encephalopathy. This study was conducted to compare the prognosis of jaundice infants with those with jaundice and hypernatremia. METHODS: A total of 615 term infants with idiopathic jaundice with or without hypernatremia were enrolled in this cohort study with 24-months follow-up at Ghaem Hospital, Mashhad, Iran, between 2010 and 2022. An in-house questionnaire including the laboratory evaluation and neonatal characteristics was used as the data collection tool. The follow-up of neonatal development status was performed using the Denver test II at 6, 12, 18, and 24 months after discharging from hospital. RESULTS: Normal outcomes were seen in 555 (90.2%) out of 615 studied infants, while 60 cases (9.8%) showed abnormal outcomes. Serum levels of sodium (P = 0.017), bilirubin (P = 0.001), urea (P = 0.024), and creatinine (P = 0.011) as well as hyperthermia (P = 0.046) and unconsciousness (P = 0.005) showed significant differences between the two groups. Approximately 16% of the newborns with both jaundice and hypernatremia, and 9% of those with only jaundice had unfavorable prognoses. Also, bilirubin level had the most predictive power (91.3%). CONCLUSIONS: Our results suggest that hypernatremia or jaundice alone, may affect the prognosis of infants aged 2 years; but jaundice and hypernatremia together, will intensify the developmental problems in jaundice infants. However, the role of hyperbilirubinemia in the incidence of complications is more than hypernatremia.