OBJECTIVE To investigate the presence of IgG4+ plasma cells in gastric mucosal biopsy samples from patients with atrophic gastritis (AG) and a history of pernicious anaemia (PA) (AG+PA+). METHODS Gastric mucosal biopsy specimens from 46 patients with AG+PA+ were investigated. As controls, we evaluated specimens from patients with AG but no history of PA (AG+ PA-) (n=25), normal histology (n=25), mild chronic inactive gastritis (MCIG) (n=25) or Helicobacter pylori gastritis (HP) (n=25). IgG4+ plasma cells were detected by two immunohistochemical methods: (1) using a monoclonal antibody, the average of the three most cellular high-power fields was counted in areas with the highest density of IgG4+ plasma cells; (2) using a dual-chromagen stain for both IgG4 and CD138 (plasma cell marker), the number of IgG4+ cells per 200 CD138+ plasma cells was counted. The latter was used to ensure that the number of IgG4+ cells was not simply related to the degree of inflammation (density of plasma cells). RESULTS Identical results were obtained with the two staining methods. Increased numbers of IgG4+ plasma cells were present in 37% of patients with AG+PA+, but in none with AG+PA-, MCIG, HP or normal gastric biopsy results (100% specific, p=0.0001). CONCLUSIONS IgG4+ plasma cells may play a role in the pathogenesis of PA and may be a useful marker for its diagnosis.