We report two cases of hypertriglyceridemic necrotizing pancreatitis treated by plasma exchange (PE). The outcome of each case was quite different according to the timing of PE. A 36 year old man presented with abdominal pain, and a diagnosis of severe acute pancreatitis was made. His serum triglyceride (TG) level was 6,460 mg/dl. He did not undergo PE at first, however, his condition never improved and PE was performed 20 days after the onset of his illness. Finally, he died of multiple organ failure and sepsis. In contrast, a 52 year old man with acute necrotizing pancreatitis was referred to our department. He received PE quickly after hospital admission. His serum TG level, which was 3,540 mg/dl at hospital admission, dramatically returned to normal limits, and he was discharged from the hospital 62 days after admission. The prognosis of severe necrotizing pancreatitis due to hypertriglyceridemia is extremely poor. PE should be applied for the treatment of hypertriglyceridemic necrotizing pancreatitis immediately after its onset.