Ultrastructural characteristics of the intracytoplasmic lumen(ICL) in the human gastric carcinoma were investigated on a total 69 histologically proven cases(42 gastric carcinomas, 15 gastric adenomas, 4 intes tinal metaplastic mucosa, 6 foveolar epithelia and 2 parietal cell-containing fundic glands) obtained by gastric endoscopic biopsy, to elucidate the relative incidence and its histogenesis. The followings were the summary; 1) ICLs were demonstrated in 38. 1% of gastric carcinoma cases, but none in gastric ade- noma, metaplastic or in foveolar epithelia. 2) Relative incidence of ICLs seemed to reflect a cellular dedifferentiation by means of disproportion to the degree of neoplastic differentiation. 3) ICLs were diffusely coated with relatively well developed microvilli, whereas their rootlet formation was poor and focal in distribution. 4) Mean number of ICL was 0. 25 per cancer cell. 5) An average tangential diameter of ICLs was 7.85um, and size ratio to cancer cell was (60.62%) These morphometric data ranged in minimal deviation with histological subtypes. 6) ICLs were demonstrable even under the light microscope, and presented the possibility to be confused with signet ring cell due to morphological similarity. 7) Ultrastructural step sections demonstrated the evidences that the ICLs often communicated with apical cell surface so as in ICL-like structures of parietal cells, and suggested their histogenesis to represent the invagination of cell surface. The above findings suggest that the ICLs in gastric carcinoma cells develop by an abnormal invagination in the process of cellular dedifferentiation and reflects the ultrastructural expression of a compensatory unicellular secretory function by expansion of the surface area.