"Crohn's Carcinoma" of a 34-year-old patient is presented in this paper. The fine structure of the tumour is discussed in detail, because there have been no previous reports of electron microscopic studies of this tumour. The adenocarcinoma cells were electron microscopically less differentiated than expected on the basis of light microscopic examinations. In the tumor cells, several round dense granules were observed. Similar granules were also described in the cells of intestinal type gastric and colon cancers, being considered a sign of pathologic mucus secretion. In a part of the tumour cells, intracisternal parallel tubular inclusions were seen like those described in malignant melanoma and osteosarcoma. In the dysplastic small-bowel mucosa adjacent to the tumour, the maturation disorder of the epithelial cells was similar to changes described in the precancerous states of the colon.