The K-ras gene is mutated in > or =75% of human pancreatic adenocarcinomas and in a number of hyperplastic ductal lesions from noncarcinoma patients. In this study, the incidence of K-ras mutation was determined in a spectrum of focal proliferative pancreatic lesions to evaluate their preneoplastic significance. PCR-based mutation-enriched RFLP analysis was used to identify mutations in codon 12. Immunostaining for Ki67 and p53 was also performed. Forty-seven % of intraductal nonpapillary hyperplasias (8 of 17) contained codon 12 mutations, as did 55% of adenomatoid hyperplasias (6 of 11). This rate increased to 61% in papillary hyperplasias (27 of 44) and to 78% when there was severe dysplasia (7 of 9). The fraction of cells staining for the Ki67 proliferation marker showed a general correlation with the rate of K-ras mutation. Nuclear staining for p53 protein was seen only in two ductal lesions with severe dysplasia. No mutations were found in normal acinar tissue (n = 38), squamous metaplasia (n = 13), ductal complexes (n = 8), or focal acinar cell dysplasia (n = 5). There seemed to be a general correlation of proliferative potential with the presence of K-ras mutation in ductal lesions. However, because of the high prevalence of lesions with K-ras mutations, we conclude that this mutation alone cannot be taken as proof of significant risk for progression to carcinoma. Efforts to use the presence of K-ras mutations in DNA harvested from pancreatic juice or duodenal aspirates as an approach for diagnosis of occult pancreatic carcinoma seem vulnerable to a high false-positive rate.