The histogenesis of pancreatic islet cell tumors was investigated by morphological identification of putative precursor lesions in pancreatic tissue from patients with multiple endocrine neoplasia type 1 (MEN1), tissue microdissection, and genetic analysis. MEN1 mutation and absence of the MEN1 wild-type allele in different precursor lesions strongly suggest that pancreatic islet cell tumors are derived from the ductal/acinar system but not from pancreatic islet tissue. Pluripotent cells within the exocrine pancreas appear capable of formation into small atypical accumulations of MEN1-deficient cells with both exocrine and endocrine phenotype. The findings suggest presence of multiple developmental aberrations in MEN1 pancreas that potentially serve as precursor material for neuroendocrine tumors.