Objectives Fulvestrant is an estrogen receptor (ER) antagonist that binds, blocks and degrades the estrogen receptor and is currently used in adjuvant treatment in postmenopausal women with ER-positive breast cancer as an alternative for tamoxifen. As an antagonist, it may induce or aggravate climacteric symptoms. In order to alleviate these symptoms, one could consider hormone therapy. The objective of this study was to analyze the effect of fulvestrant alone or in combination with different steroids in human breast cancer cells invitro, and to demonstrate whether these steroids will compromise the efficacy of fulvestrant in ER-positive breast cancer cells. Methods We performed experiments invitro with various hormone therapy preparations (estradiol (E2), dihydrodydrogesterone (DHD) and tibolone) at a concentration of 10(-6) mol/l alone or combined with fulvestrant in different breast cancer cell lines, ER-positive and ER-negative. After an incubation of 144h, proliferation and apoptosis were measured. The first was measured by quantification of the expression of cyclin D1 mRNA, the latter by the Nicoletti fragmentation assay. Results This invitro study revealed clear differences in results when various hormone therapy preparations, alone or combined with fulvestrant, are added to ER-positive and ER-negative breast cancer cell lines. Conclusions Our study demonstrated that fulvestrant, an ER antagonist used in the treatment of ER-positive breast cancer, combined with E2 and DHD or in combination with tibolone, is not compromised in its efficacy in inducing apoptosis in ER-positive breast cancer cell lines invitro.