Hypotonic fluids are widely used in pediatrics. Several articles have reported the risk of iatrogenic hyponatremia secondary to this practice. We primarily intend to determine whether the use of isotonic fluids prevents hyponatremia and, secondly, whether these fluids increase the incidence of adverse events. One hundred twenty-two pediatric patients hospitalized in intensive care unit requiring maintenance fluid therapy were randomized to receive isotonic fluids (isotonic group, NaCl = 140 mEq/L) or hypotonic fluids (hypotonic group, NaCl <100 mEq/L). Electrolyte blood concentration, glycaemia, and blood pressure were measured at 0, 6, and 24 hrs after the beginning of fluid therapy. Plasma creatinine, urine specific gravity, and urine electrolyte concentration were measured at 6 hrs. Standard intention-to-treat analysis and Bayesian analysis were conducted to assess the probability of hyponatremia and hypernatremia in each group. At the time of admission to hospital, no differences in natremia or the percentage of hyponatremia were found between groups. At 24 hrs, the percentage of hyponatremia in the hypotonic group was 20.6% as opposed to 5.1% in the isotonic group (p = 0.02). No differences in the number of adverse events other than hyponatremia were observed between groups. The use of hypotonic fluids increases the risk of hyponatremia when compared with isotonic fluids at 24 hrs following infusion (number needed to harm [confidence interval 95%] = 7[4;25]). In our sample, the use of isotonic fluids did not increase the incidence of adverse events compared with hypotonic fluids.