The presence of autoimmune gastritis was investigated in 54 women with postpartum thyroiditis. Parietal cell antibodies (PCA) specific against H+, K(+)-adenosine triphosphatase (EC 3.6.1.36) were found in 18 women during pregnancy; in 10 of them, a 2-9-fold increase in the PCA level was observed in the postpartum period. At a 5-year follow-up, the initially PCA-positive women still had elevated antibody levels. Hypergastrinemia and low pepsinogen levels were noted in 4 women. In 2 of these women low serum vitamin B12 levels had developed. In 6 of 9 PCA-positive women examined by gastroscopy, biopsy specimens from the gastric body mucosa contained mononuclear cells, mainly T lymphocytes (CD3+) and macrophages (Leu-M3+) combined with an aberrant epithelial expression of HLA-DR. In four patients with chronic gastritis, all parietal cells, as defined by a specific monoclonal antibody, were found to have immunoglobulin G (IgG) deposits by a double-immunostaining method. Three of them had microscopic evidence of atrophy, whereas in 1 patient the body mucosa was intact. In 1 further patient with intact glands at histological examination, the basolateral membrane of some oxyntic glands was coated with IgG. The selective in situ deposition of antibodies associated with histologically intact parietal cells may support the concept that specific autoantibodies participate in the early pathogenesis of parietal cell destruction.