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Background. In children, hypernatraemia occurs most commonly in infants (younger than 1 year). Although hypernatraemia is associated with high mortality and morbidity rates, it is variably defined in the paediatric literature as either serum sodium ≥150 mmol/L or serum sodium >145 mmol/L. In hospitalised adults, a serum sodium level >145 mmol/L but <150 mmol/L (called borderline hypernatraemia) has recently been identified as an independent risk factor for mortality. There are limited data about a potential association between borderline hypernatraemia and mortality in infants. Objectives. To determine whether borderline hypernatraemia is associated with increased mortality in hospitalised infants. Methods. We conducted a single-centre, retrospective observational study of 8 343 infants admitted to a tertiary-level academic hospital in Johannesburg, South Africa, of whom 254 had borderline hypernatraemia, 376 had hypernatraemia (serum sodium ≥150 mmol/L), and 7 713 did not have hypernatraemia. Mortality rates were reported as odds ratios (ORs) with 95% confidence intervals (CIs). Results. In 254 infants with borderline hypernatraemia, 115 (45.3%) were neonates (≤28 days old) and 140 (55.1%) were male. In 139 infants >28 days old with borderline hypernatraemia, the mortality rate (n=9/139; 6.5%) was significantly higher than the mortality rate observed in infants without hypernatraemia (n=194/5 857; 3.3%) (OR 2.02; 95% CI 1.03 - 3.98). Conclusion. Borderline hypernatraemia may be a risk factor associated with higher mortality in hospitalised infants. Prospective studies are required to determine whether borderline hypernatraemia contributes independently to mortality risk in hospitalised infants. [ABSTRACT FROM AUTHOR]