Total parenteral nutrition (TPN) is a revolution in neonatal intensive care unit (NICU) care, but this therapy is not without problems. A 35-week-old, 1300 g female infant was transferred to our NICU because of bilious vomiting and feeding problems. When enteral feeding was started again, a severe condition similar to the previous one developed. On the 24th day, the patient underwent surgery with a diagnosis of Hirschprung's disease. One week before surgery, the parenteral solutions were composed without vitamins because intravenous vitamin supplements suitable for infants were not available. Thereafter, the patient suffered from severe hypoglycaemia, and sepsis started to develop, accompanied by a large anion gap and metabolic acidosis which is severe lactic acidosis refractory to massive doses of bicarbonate. The acidosis improved significantly when the patient was treated with thiamin. Although TPN is life saving in the NICU, meticulous attention must be paid while treating a patient with TPN, and all possible nutrients should be provided. In this report, a case of a preterm newborn requiring a prolonged period of TPN and complicated by serious lactic acidosis is presented and discussed.