The incidence and types of unusual nuclear structures (UNS) were examined based on the observation of randomly photographed 1,919 nuclei in total from the normal epithelium, dysplasia, Peutz-Jeghers type polyp, adenoma and carcinoma of the large bowel. These nuclear structures were classified as filamentous, granular and vesicular inclusions, and invaginated structures were also taken into consideration. Although all types of UNS were found in the normal epithelium, carcinoma cells had the most numerous UNS, especially of the invaginated type. The possible process of nuclear inclusion (NI) formation was discussed in connection with malignant transformation of cob-rectal epithelial cells.nuclear inclusions ; cob-rectal neoplasm ; dysplasia ; hamartoma Since the description by de The et al. (de The et al. 1960;Kera et al. 1979Kera et al. , 1980 nuclear bodies have been well documented. Nevertheless, nuclear inclusions (NIs) other than nuclear bodies have not been sufficiently reported except for neural tissues (Masurovsky et al. 1970; Vazquez et al. 1970; Tani 1971; Shaw and Sumi 1975) and endocrine organs (Boquist 1969;Dahl 1970; Sherwin et al. 1977;Ryder et al. 1979;DeLellis et al. 1980). Through a systematic observation of several polypoid lesions and carcinomas of the large bowel, we found a number of nuclear inclusions. As the nuclear inclusions in the colonic epithelium have not been described to date, a detailed examination was undertaken, and special attention was focused on neoplastic changes of colonic epithelium in relation to nuclear invagination and the formation of nuclear inclusions.