It is important but difficult to differentiate pancreatic cancer without mass formation, chronic pancreatitis and pancreatic cancer coexisting with chronic pancreatitis. These diseases have similar clinical presentations and markedly overlapping findings in various imaging studies, but have extremely different prognosis and treatment modalities. We report a case of pancreatic head cancer without mass formation. Its clinical features are similar to chronic pancreatitis in the initial studies. However, it turned out to be malignant after 4 months. (Kor J Gastroenterol 1998;32:588-694)