AIMS: To assess the prevalence of gastric giardiasis in patients undergoing upper gastrointestinal endoscopy, and to define the clinicopathological correlates of gastric Giardia lamblia infection. METHODS: Consecutive gastric biopsy specimens (n = 15,023) from 11,085 patients, taken at Feltre City Hospital (north eastern Italy) from January 1986 to December 1991, were histologically and immunocytochemically examined for the occurrence of G lamblia trophozoites. Three gastric biopsy specimens from patients harbouring G lamblia infection, who repeated endoscopy before treatment, were also examined electron microscopically. RESULTS: Forty one patients (0.37% of the population study) harboured gastric giardiasis. All patients underwent upper gastrointestinal endoscopy because of dyspepsia, epigastric pain, or abdominal distension. Only two patients had diarrhoea at the time of investigation. Giardiasis was clinically unsuspected in all cases, although the nine patients who also had duodenal biopsies performed had concomitant intestinal giardiasis. Gastric giardiasis was invariably associated with chronic atrophic gastritis. Intestinal metaplasia of the gastric mucosa and Helicobacter pylori infection were found in 32 and 37 of the 41 patients with gastric giardiasis, respectively. CONCLUSIONS: The invariable association of gastric giardiasis with chronic atrophic gastritis, most often showing intestinal metaplasia and H pylori infection, indicates that a decreased gastric acidity is a prerequisite for localisation of G lamblia to the gastric mucosa. Though its possible role as a gastric pathogen remains to be elucidated, these findings suggest that trophozoites should be carefully searched for when examining gastric biopsy specimens showing chronic atrophic gastritis.