Thirty-six cases of pancreatic cancer including 27 cases of so-called mucin producing cancer of the pancreas, examined with endoscopic ultrasonography (EUS) and treated with surgery, were reviewed to evaluate the usefulness of EUS in the diagnosis of portal venous (PV) invasion. The detection rate of pancreatic tumors cancer by EUS was 92% in 36 cases. The detection rate of PV invasion by EUS was compared with histological results in each case. Judgement of the correlation between the tumor and the PV wall was possible in 30 cases (83%), and the overall accuracy of EUS in the diagnosis of PV invasion was 73%. It is concluded that EUS is a very useful method for diagnosis of PV invasion in cases of pancreatic cancer.