A multistep model of carcinogenesis has recently been proposed for pancreatic ductal adenocarcinomas. In this model, noninvasive precursor lesions in the pancreatic ductules accumulate genetic alterations in cancer-associated genes eventually leading to the development of an invasive cancer. The nomenclature for these precursor lesions has been standardized as pancreatic intraepithelial neoplasia or PanIN. Despite the substantial advances made in understanding the biology of invasive pancreatic adenocarcinomas, little is known about the initiating genetic events in the pancreatic ductal epithelium that facilitates its progression to cancer. Telomeres are distinctive structures at the ends of chromosomes that protect against chromosomal breakage-fusion-bridge cycles in dividing cells. Critically shortened telomeres can cause chromosomal instability, a sine qua non of most human epithelial cancers. Although evidence for telomeric dysfunction has been demonstrated in invasive pancreatic cancer, the onset of this phenomenon has not been elucidated in the context of noninvasive precursor lesions. We used a recently described in situ hybridization technique in archival samples (Meeker AK, Gage WR, Hicks JL, Simon I, Coffman JR, Platz EA, March GE, De Marzo AM: Telomere length assessment in human archival tissues: combined telomere fluorescence in situ hybridization and immunostaining. American Journal of Pathology 2002, 160:1259-1268) for assessment of telomere length in tissue microarrays containing a variety of noninvasive pancreatic ductal lesions. These included 82 PanIN lesions of all histological grades (24 PanIN-1A, 23 PanIN-1B, 24 PanIN-2, and 11 PanIN-3) that were selected from pancreatectomy specimens for either adenocarcinoma or chronic pancreatitis. Telomere fluorescence intensities in PanIN lesions were compared with adjacent normal pancreatic ductal epithelium and acini (62 of 82 lesions, 76%), or with stromal fibroblasts and islets of Langerhans (20 of 82 lesions, 24%). Telomere signals were strikingly reduced in 79 (96%) of 82 PanINs compared to adjacent normal structures. Notably, even PanIN-1A, the earliest putative precursor lesion, demonstrated a dramatic reduction of telomere fluorescence intensity in 21 (91%) of 23 foci examined. In chronic pancreatitis, reduction of telomere signal was observed in all PanIN lesions, whereas atrophic and inflammatory ductal lesions retained normal telomere length. Telomere fluorescence intensity in PanIN lesions did not correlate with proliferation measured by quantitative Ki-67-labeling index or topoisomerase IIalpha expression. Thus, telomere shortening is by far the most common early genetic abnormality recognized to date in the progression model of pancreatic adenocarcinomas. Telomeres may be an essential gatekeeper for maintaining chromosomal integrity, and thus, normal cellular physiology in pancreatic ductal epithelium. A critical shortening of telomere length in PanINs may predispose these noninvasive ductal lesions to accumulate progressive chromosomal abnormalities and to develop toward the stage of invasive carcinoma.