We report a novel abnormality affecting simple mucin-secreting cells of the glands of pyloric gastric mucosa and of Brunner's glands. Subnuclear vacuolated mucous cells show a distinctive appearance on haematoxylin and eosin staining. They are columnar cells of similar size to simple mucin-secreting cells, but have a central nucleus beneath which the cytoplasm has a uniform glassy eosinophilic appearance or contains a clear area. Subnuclear vacuolated mucous cells are found focally lining the lower third of pyloric gastric glands or in Brunner's glands, and may be mistaken for a form of metaplasia. Histochemically, the apical portion of the cell stains for neutral mucin, but the basal portion stains only weakly and variably for protein. Electron microscopy reveals that the basal portion consists of a large, single, membrane-bound vacuole, variably indented by the nucleus, and probably derived from either endoplasmic reticulum or the Golgi apparatus. The vacuole contains granular material which varies in electron density from cell to cell. The apical portion of the cell contains organelles similar to simple mucin-secreting cells. These appearances are highly suggestive of an abnormal accumulation of non-glycoconjugated mucus core protein. Although the cause of this is unknown, it was associated with the histological changes of chronic gastritis in 10 of our 12 cases.