A three-dimensional analysis of the histogenesis of gastric cancer was made by examining minute, microscopic lesions of less than 500 mu. Cancer develops from the generative cell zone (G-zone) of the atrophic gastric glands and intestinal metaplastic glands. They then bud or branch and infiltrate into the propria interstitial tissue. The G-zone of the cancerous glands showed marked expansion with complete loss of normal cell kinetics of gastric glands and with hardly any signs of differentiation into pyloric glands. In the adjacent glands there were also expansion of the G-zone and formation of loop-shaped glands with abnormal branching and anastomosis sometimes leading to the formation of a network of glands. These changes are compatible with those of abnormal differentiation accompanying atrophy or intestinal metaplasia and are considered to express a preinvasive cancer.