Hypoplastic left heart syndrome (HLHS) is a congenital heart defect that requires 3-stage cardiac surgical treatment and multidirectional specialist care. The condition of newborns in the first postoperative days following the modified Norwood procedure is characterized by considerable hemodynamic instability that may result in a sudden cardiac arrest. It is believed that the most important cause of hemodynamic instability is the fluctuations in redistribution between pulmonary and systemic blood flow.The paper analyzes the postoperative course in 40 neonates with HLHS following the modified Norwood procedure performed under deep hypothermic cardiopulmonary bypass hospitalized in Cardiac Surgical Intensive Care Unit (CSICU) in the years 2014-2015. For all hospitalized children, the arterial blood acid-base balance (ABB) parameters (pH, pO2, pCO2, HCO3, base excess (BE), and lactic acid) were measured 2 times a day during the first 5 postoperative days. The main goal of the studies is to analysis of ABB parameters and their influence on the clinical state of newborns with HLHS. Several descriptors were concerned to describe the neonates clinical state: the date of the surgery (the day of life when the child was operated on), the duration (number of days) of mechanical ventilation employment, the time of hospitalization in intensive care unit, and the total duration of treatment in CSICU.The statistical analysis of the particular ABB parameters revealed a significant dependence (P < .001) between the values of pH, pO2, pCO2, HCO3, BE, lactic acid, and all concerned descriptors of the newborn clinical state.The article shows that monitoring the ABB parameters, proper interpretation of the results, and appropriate modification of pharmacotherapy and respiratory treatment are crucial for therapeutic results and survival rates in neonates with HLHS after the modified Norwood procedure.