The prevalence of Kirsten (Ki)-ras gene mutations was studied in 105 paraffin-embedded tissues obtained from 40 patients with pancreatic cancer, 48 with bile duct carcinoma (19 distal, 6 middle, and 23 proximal), 16 with ampullary carcinoma and 1 with duodenal cancer, by in vitro amplification of target sequences by the polymerase chain reaction (PCR). With regard to pancreatic cancers, the authors' data confirm the very high frequency (88.6%) of Ki-ras gene mutations occurring at codon 12. Five pancreatic carcinomas did not contain the Ki-ras mutation and included rare types of histopathology. By histologic review after the examination of Ki-ras mutations through PCR, the diagnosis of four patients could be legitimately revised from other periampullary carcinoma to pancreatic carcinoma. In the ampullary carcinoma, the prevalence of mutations in Ki-ras codon 12 was 13.3%. Although there was a large difference in incidence of mutations between distal and middle or proximal bile duct carcinoma, the prevalence of mutations in bile duct carcinoma was limited to 19.6%. Unlike other approaches to diagnose periampullary carcinoma, detection of a mutation in Ki-ras codon 12 by PCR may distinguish pancreatic carcinoma from other periampullary carcinomas that have better prognoses.