We report a highly frequent homozygous deletion of the p16/CDKN2 gene in the esophageal cancer cell line and a relatively high frequency of homozygous deletion in gastric cancer cell lines. In contrast, in primary esophageal carcinomas, mutation frequency of the p16/CDKN2 gene has been controversial (0, 21, and 52% previously reported), and no reports are available for the mutation frequency of this gene in surgical specimens of gastric carcinomas. Here we report that four (16%) of 25 primary esophageal squamous cell carcinomas were found to be mutated, one in exon 1 and three in exon 2, and that no mutations were observed in 19 surgical specimens of gastric adenocarcinomas. This is the first report showing the absence or quite low frequency of mutation in surgical specimens of gastric carcinomas.