The morphology of pancreatic endocrine tumors (PETs) is similar to that of endocrine tumors elsewhere in the body. PETs are usually encountered in adults. They may be clinically functional and associated with various syndromes related to hormone excess. However, it must be remembered that absence of obvious clinical symptoms may not necessarily reflect true lack of clinical function, and subtle clinical manifestations may be missed. Current thinking indicates that PETs arise from totipotential stem cells as well as preexisting endocrine cells. PETs may be hereditary or sporadic. The hereditary forms are associated with multiple endocrine neoplasia type 1 (MEN-1), von Hippel-Lindau syndrome, neurofibromatosis, and tuberous sclerosis. In sporadic PETs, the most consistent and recurring chromosomal abnormality is allelic loss of chromosome 11q, which includes the MEN-1 locus. Loss of a sex chromosome has been shown to be associated with metastasis, local invasion, and poor survival.