Cardiac tamponade occurs very rarely, but is life-threatening in the newborn. This paper reports a premature newborn who developed profound shock 25 hours after undergoing umbilical venous catheterization. Echocardiography taken later, showed marked pericardial effusion. An umbilical venous catheter was located in the left atrium. Immediate pericardiocentesis was performed, 11 mL of a clear straw-colored fluid was removed and the umbilical venous catheter was withdrawn into the inferior vena cava. The heart rate and blood pressure recovered immediately. Analysis of the pericardial fluid showed a high glucose level of 2,451 mg/dL. There was no pericardial effusion reaccumulation thereafter. Rapid diagnosis and treatment of pericardial effusion are mandatory to prevent subsequent morbidity and mortality when disastrous episodes, such as in the present case, occur.