Leukemia inhibitory factor (LIF) is a pleiotropic inflammatory cytokine. A potential role for LIF in the pathogenesis of human breast cancer was recently indicated by the finding that LIF is produced by MDA-MB 231 breast cancer cells and that it stimulates proliferation of the T47D and MCF-7 breast cancer cell lines. Despite its role as a possible therapeutic target in breast cancer, the transcriptional regulation of the LIF gene in breast cancer cells has not been investigated so far. In this context, we investigated the regulation of the human LIF promoter (human LIF666-luciferase) by ovarian steroids in transient transfection assays in MDA-MB 231 and T47D cells. Since the MDA-MB 231 cells are devoid of both estrogen (ER) and progesterone (PR) receptors, these cells were co-transfected with the respective receptor expression vector. Estradiol induced no stimulation in either T47D or ER-transfected MDA-MB 231 cells. Treatment with the progesterone agonist MPA (medroxy-progesterone acetate) resulted in induction of LIF transcription in PR-transfectant MDA-MB 231 cells, while it had no effect in T47D cells. Both PR isoforms (PR-B and PR-A) were effective in inducing the LIF promoter in MDA-MB 231 cells, and this effect was inhibited by the progestin antagonist RU 486. The stimulatory effect of MPA was maintained on deletion constructs (h274LIF-Luc, h148LIF-Luc and h82LIF-Luc), indicating that 82 bp are sufficient to mediate this effect. Our results indicate that the LIF promoter is transcriptionally active in human breast cancer cells and its activity can be modulated by progestins and anti-progestins in cells expressing the LIF protein, which might have therapeutic implications.