The reverse transcription polymerase chain reaction (RT-PCR) was performed to detect genes of RNA viruses in the freshly biopsied gastric mucosa of seven patients with low gastric acidity. Although nucleoprotein genes of Sendai virus and hemmaglutinin genes of influenza virus A were not detected, nucleoprotein genes of influenza virus B were detected in samples from three of the seven patients. The first patient had had antrectomy and vagotomy for gastric ulcer, the second patient was receiving a histamine type 2 receptor blocker for gastritis, and the third patient was receiving a proton pump inhibitor for gastric ulcer. Virus isolation from gastric mucosa and from gargles was negative for all seven patients. These findings suggest that genes of influenza viruses may exist in the gastric mucosa of patients with low gastric acidity.