Seven‐hundred‐and‐sixty‐four stomachs from the Japanese and 456 from the Americans, both autopsy and surgical materials inclusive, were studied histologically. There were seen, within an unexpectedly large proportion of cases, distortion and thickening of the muscularis mucosae. This change consisted of a proliferation of fibrous and muscular elements. The incidence of the lesion increased with the advance of age reaching 77.4 per cent in the 6th decade in autopsy cases in the Japanese, whereas in the Americans it was less than 20.0 per cent, thus making a sharp contrast. Its association with classical gastritic changes was a usual occurrence. The term “fibromusculosis” was proposed by the author for the lesion, on the ground that it would constitute one of the important anatomical features of chronic “Umbau‐gastritis”