A 43-year-old woman was referred to our hospital for cardiomegaly on chest X-ray (Fig. 1). Physical examinations revealed a healthy appearing woman except for distant heart sounds. All laboratory findings were within the normal range. The patient had no history of trauma, congenital heart disease, thoracic surgery or tuberculosis. Transthoracic echocardiography revealed a large amount of pericardial effusion without hemodynamic compromise. Diagnostic pericardiocentesis was performed with 500 mL of a milky-colored pericardial fluid aspirated. Routine examination of the pericardial fluid revealed pH 6.8, protein 5.0 g/dL, and LDH 234 IU/L. WBCs and RBCs were uncountable due to cell clumping. Routine bacterial and tuberculous cultures were subsequently reported as negative, as was the cytological examination, and the adenosine deaminase level was 15.5 U/L. Considering these clinical and laboratory findings, the initial clinical impression was of nonspecific chronic pericarditis with effusion. We did not diagnose chylopericardium because we overlooked the clue offered by the color of the pericardial fluid. After pericardiocentesis, follow-up echocardiography revealed that minimal pericardial effusion remained. She was regularly followed up at our outpatient clinic after discharge. However, echocardiography one month later revealed a large amount of pericardial effusion without evidence of hemodynamic compromise (Fig. 2), and she was readmitted to our hospital for further evaluation. The patient, who was afebrile on admission, complained only of mild substernal discomfort. Her blood pressure was 90/60 mmHg and pulse rate 72 beats per minute. On physical examination neck veins were distended and peripheral pulses were normal. Her breathing sound was clear without rale, though heart sounds were slightly decreased and pericardial friction rub was inaudible. The chest X-ray showed marked enlargement of the cardiac shadow with a globar shape, and an electrocardiogram demonstrated a sinus rhythm, with low voltage QRS complexes in the precordial leads. The laboratory examination showed; white cell count 7,740/mm3, hemoglobin 13.0 mg/dL, hematocrit 39%, and platelets 238,000/mm3. The ESR was 17 mm/hr and CRP 6.7 mg/dL. Blood chemistry revealed; BUN 12.9 mg/dL, creatinine 0.6 mg/dL, total protein 7.0 g/dL, albumin 4.7 g/dL, AST/ALT 21/17 IU/L, total cholesterol 196 mg/dL, triglyceride 111 mg/dL, HDL-cholesterol 46 mg/dL, and LDL cholesterol 138 mg/dL. Urinalysis was normal. Antinuclear antibody and anti-viral antibodies including HIV were all negative. beta2-microglobulin and CEA were within the normal range. Repeated pericardiocentesis was performed, and an immediate gushing out of 700 mL of a milky-yellowish fluid ensued (Fig. 3). An examination of the pericardial fluid showed pH 6.8, protein 7.0 g/dL, glucose 98 mg/dL, LDH 560 IU/L, cholesterol 97 mg/dL, and triglyceride 1078 mg/dL. RBCs and WBCs were uncountable due to cell clumping. The adenosine deaminase level was 17.0 U/L and the CEA level 1.15 ng/mL. Routine cultures for bacteria and tuberculosis were subsequently reported as negative and a cytologic examination showed no tumor cells. These findings with inappropriately elevated triglyceride in the pericardial fluid sufficed to diagnose chylopericardium. The patient underwent extensive investigations to determine the secondary causes. Chest computed tomography revealed a suspicious small cystic lesion on the left neck and mediastinal haziness. The possibility of cystic hygroma with mediastinal extension was highly suggested by radiologists. Neck sonography also showed a 1.9 cm sized multilocular, compressible cystic lesion between the sternocleidomastoid muscle and the internal jugular vein of the left neck. These findings were compatible with cystic hygroma (Fig. 4). The cystic lesion could not be aspirated due to patient's refusal. To demonstrate the presence of any anomalous communication between the lymphatic channels and the pericardial sac, we performed a direct lymphangiography. However, no direct communication between the pericardial sac and the thoracic duct could be definitely established. Follow-up echocardiography was performed 10 days after the pericardiocentesis and showed a massive reaccumulation of pericardial effusion. 11 days after pericardiocentesis, the patient underwent pericardiostomy. Using the subxyphoid approach, a pericardial window was made with 32 Fr draining tube insertion in an attempt to prevent pericardial fluid reaccumulation and cardiac tamponade. Incision of the pericardium resulted in the extrusion of 600 mL or more of milky fluid. The pericardial fluid analysis showed that it had the same nature as the previous sample. When the patient was placed on total parenteral nutrition for 30 days and a subsequent low fat diet enriched with medium-chain triglycerides for 15 days, the drainage reduced to 30-40 mL per day. On the 45th day after pericardiostomy, drainage tube was removed and she was discharged from the hospital on a low fat diet. She remained totally asymptomatic without interval changes of heart size on chest X-ray (Fig. 5) or clinical signs of cardiac tamponade at the 6 month follow-up.