Using immunohistochemical techniques, we studied the expression of c-erb B-2 in normal human endometrial tissue (n = 8), endometriosis interna (adenomyosis) (n = 8) and endometriosis externa (endometriotic cyst of the ovary) (n = 6). The glandular epithelium of normal endometrial tissue specimens in the proliferative phase stained positively. Most of the cases of endometriosis studied showed no expression of c-erb B-2 in glandular epithelium. No expression was detected in either the normal or endometriotic stromal cells. Results suggest that c-erb B-2 is not significantly involved in the pathogenesis of endometriosis.