Studies have shown that the application of semipermeable membranes to the skin of premature newborns (NBs) can aid in protecting the skin, reduce disturbances in fluid and electrolyte levels, and decrease neonatal mortality. The aim of this study was to verify the effect of using semipermeable membranes in low-birth-weight preterm newborns (PTNBs). A randomized controlled trial was carried out in the neonatal intensive care unit (NICU) with 42 NBs split evenly into an intervention group (IG), in which semipermeable membranes were used to cover large areas of the skin for the first 7 days of life, and a control group (CG), which received normal care. The variables investigated for the study were weight, hydration status, urinary density, glycemic control, sodium concentration, and daily hydration quota. The following variables displayed significant daily variation: weight, hydration quota, and sodium concentration. Statistically significant individual effects by day and by group were found only for sodium concentration. In the overall analysis of the intersubject effects, sodium concentration, alone, proved to be significant (p = .055). Significant effects by group in relation to the sodium concentration were found, with the IG showing a lower average sodium concentration than the CG. Thus, the use of semipermeable membranes reduced fluid loss in premature NBs in the current study, confirming the findings of previous studies. Guidelines for practice may now be warranted.