Anemia is defined as a fall in oxygen delivery to the tissues as a main consequence of a decrease in the circulating hemoglobin concentration. It is frequently observed in NICU patients, specially in preterm very low birth weight infants (VLBW) who have decreased bone marrow response. The decision for a transfusion of packed red blood cells is made based on relatively unspecific parameters such as tachycardia, taquypnea, absence of weight gain or apnea, besides on the objective hematocrit (HC) founds. However, the exact grade of hypoxia cannot be established with the sole measurement of the HC. If their is a mismatch between oxygen delivery and oxygen requirement, metabolism of glucose may use the anaerobic pathway. This is less efficient and produces a rise in lactic acid (LA) levels: In this context, lactacidemia could be an indicator of the need for transfusion. Normal values in adults range from 1.0 to 1.8mmol/l (9-16 mg/dl). Although normal levels have not been established in healthy newborns, data on the rise of LA in talasemias, and iron deficiency anemia in adults, in severe hemorrhage in animals, and their fall after transfusion have been reported. The present study was designed to determine a) LA levels in low birth weight) (LBW) infants before and after a transfusion of packed red blood cells and its correlation with HC. B) the existence of significant differences between pre and post-transfusional values, and c) if LA level can be used as an indicator for anemia and the need for transfusion. Ten LBW (< 2000gr) infants hospitalized in the NICU in whom a transfusion has been indicated for the treatment of anemia were included. Arterial blood samples of 0.2ml were drawn pre and post- transfusion and HC and LA levels were compared. LA was measured using an enzymatic assay. Mean pre and post-transfusion values for LA were 3.10mmol/l (1.2-9.2) and 1.75mmol/l (0.65-4.85), respectively. The mean of the differences was 1.346 (p=0.009). Mean pre and post-transfusional HC values were 24.9% (21-31) and 32.3% (24-39), respectively, with a mean of the differences of 7.44. These data show a significant difference between pre and post-transfusional LA levels. These latter were consistently lower than the former. However, levels had no correlation with HC values. However, due to the great variability in LA levels, found in this group of patients, lactacidemia cannot be used as an indicator of the need for transfusion in the newborn with low HC.