Fourteen cases of gastric cancer, 9 cases of gastric polyp and 4 cases of gastric ulcer were studied on their tissues lipid-chemically with the aim to clarify the biochemical differences between malignant and benign growth of the human gastric tissue. Tissues were collected by surgical operation or a biopsy together with the normal tissues surrounding the lesions. One part of each tissue was subjected to histologic examination and the other part to lipid analyses by means of TLC and GLC. The total lipid content was dcreased in 71% of gastric cancerous cases and in 75% of both cases of gastric polyp and gastric ulcer as compared with the respective control tissues. The phospholipid fatty acid content of the lesions was decreased in 83% of cancerous cases but in 50% of polypous cases as compared with the respective control tissues. As for the phospholipid fatty acid composition, the percentage value of C20:4 was increased and the percentage value of C18:2 was decreased in gastric cancerous tissues when compared with those of gastric polypous tissues. Such a change of the phospholipid fatty acid composition may produce the change of the fluidity of the cell membrane and may provoke a malignant growth of human gastric cancer cells.