The expression and the distribution of the c-myc oncogene product (p62) was studied by a 3-step immunoperoxidase technique using the monoclonal antibody myc 1-6 E10 in 22 cases of normal endometrium (11 proliferative and 11 secretory phase), 43 endometrial hyperplasias (24 adenomatous and 19 adenocystic) and 26 endometrial carcinomas. Increased expression of c-myc product appeared in endometrial carcinomas compared with respective non-neoplastic tissue (p < 0.001). The immunolocalization of the c-myc protein shows a consistent difference between the various histologic patterns of non-neoplastic and neoplastic endometrium. Nuclear staining of the c-myc product was demonstrated in epithelial cells of the proliferative phase and predominantly in poorly differentiated forms of endometrial carcinomas. On the other hand cytoplasmic staining was found predominantly in the secretory phase and in well differentiated carcinomatous endometrium. In hyperplastic endometrium an intermediate immunohistochemical pattern was observed. The results of the present study emphasize that c-myc product overexpression and localization plays an important role in initiation, differentiation and progression of endometrial carcinomas.