We analyzed pancreatic endocrine tumors (PETs) from 200 patients for the incidence of multihormonality and entrapped islets and correlated the results with clinicopathological features. Our series included 86 cases (43%) of functioning PET and 114 cases (57%) of nonfunctioning PET. Classified according to the WHO classification, there were 32 well-differentiated benign PETs, 85 well-differentiated PETs with uncertain behavior, and 83 well-differentiated malignant PETs. All tumors were immunostained for pancreatic hormones (insulin, glucagon, somatostatin, and pancreatic polypeptide) and for additional hormones such as gastrin, vasoactive intestinal polypeptide, calcitonin, seratonin, and adrenocorticotropic hormone. Multihormonality was found in 34% of all PETs and it was a frequent finding in the tumors of the uncertain behavior (38.8%) group. Islet entrapment was found in 57 tumors (28.5%) and was significantly more frequent in PETs with uncertain and malignant behavior than benign ones (p=0.01). In 57 cases, we also investigated whether ductule entrapment accompanied islet entrapment. Of these 57 tumors, 45 (79%) tumors had accompanying ductule entrapment. Ductule entrapment did not show significant correlation with malignancy and was a more frequent finding in nonfunctioning tumors. We conclude that the incidence of multihormonality in PETs is not as high as suggested previously and islet entrapping may reflect aggressive tumor growth and may be a complementary criterion for predicting the biological behavior of PETs.