Maintaining sodium balance in the VLBW infant is not straightforward. Several predisposing factors appear in more than one imbalance constellation. In addition, the same patient circumstance can produce two types of imbalance. Understanding the physical properties of electrolytes and water enables critical care nurses to anticipate outcomes of specific situations in the neonatal ICU. Careful assessment of renal function, urine output, and water balance are crucial in determining proper treatment of sodium balance disorders. The high potential risk of errors in the neonatal ICU requires verification of all fluid and electrolyte orders, as well as serial safety checks of IV fluids and additives to avoid complications. An expert nurse's ability to predict the needs of VLBW infants places the nurse in a pivotal position to recognize subtle changes before actual physiologic change occurs. Although the multiple variations can be confusing to the bedside clinician, sorting through them helps in selecting interventions that are timely and improve the outcomes and morbidity of our smallest patients at risk.