To clarify whether changes in bcl-2 protein (bcl-2) expression are directly linked to differentiation, an immunohistochemical investigation was carried out on areas of squamous differentiation within 38 endometrial carcinomas (25 grade 1 and 13 grade 2 cases) as well as eight grade 1 carcinomas after progesterone therapy. Such areas of altered differentiation were subdivided into foci of squamous metaplasia (Sq-M) and morules. Expression of oestrogen and progesterone receptors (ER and PR) and cell proliferation were also examined. Immunoreactivity scores for bcl-2 in Sq-M foci were significantly lower than in the surrounding tumour regions, in line with reduced staining for ER and PR and Ki-67 labelling indices (LIs), while morule values were intermediate. After progesterone therapy, a marked decrease in bcl-2 immunoreactivity was found in the response group, along with low Ki-67 LIs and tumour cell maturation. These results indicate that in endometrial carcinoma, down-regulation of bcl-2 expression may be closely linked with squamous differentiation, in line with changes in ER and PR expression, as well as in cell proliferation. In addition, bcl-2 expression appears to be down-regulated by progesterone therapy through tumour cell maturation, indicating that bcl-2 may be a clinically useful marker of hormone therapy effects.