Patients harboring germline BRCA2 mutations are at an increased risk of developing pancreatic cancer. We investigated the prevalence of biallelic inactivation of BRCA2 in the presumed precursors to invasive pancreatic ductal carcinomas, pancreatic intraepithelial neoplasia (PanIN). Surgical resection specimens from three patients with germline BRCA2 mutations who developed pancreatic ductal adenocarcinoma were studied. Fourteen PanINs were needle-microdissected from paraffin-embedded tissue. DNA was isolated from these microdissected tissues and amplified by primer-mediated pre-amplification. Loss of heterozygosity at the BRCA2 locus was determined by polymerase chain reaction amplification and cycle sequencing. The presence of the wild-type alleles was evaluated at the nucleotide positions of the germline BRCA2 mutations. The K-ras gene was sequenced at codon 12 and 13 to confirm the efficacy of microdissection. By histological evaluation the prevalence of PanINs in these patients was not notably elevated. Loss of the wild-type allele of BRCA2 was present in one high-grade PanIN (PanIN 3), but in none of 13 low-grade PanINs (PanIN 1). In contrast, K-ras mutations were detectable in 7 of the 14 PanINs. These results suggest that biallelic inactivation of the BRCA2 gene is a relatively late event in pancreatic tumorigenesis. In contrast to classical molecular progression models of tumorigenesis, the inactivation of the wild-type allele in a carrier of a recessive tumor susceptibility gene may not always be the first somatic event during the molecular evolution of a cancer. The necessity for earlier genetic alterations before biallelic inactivation of a recessive tumor susceptibility gene such as BRCA2 may explain why affected carriers have normal numbers of neoplastic precursor lesions, a relatively low phenotypic penetrance, and late age of onset of pancreatic and other cancers.