Context: Type 1 17βhydroxysteroid dehydrogenase (17βHSD-1), responsible for generating active 17βestradiol (E2) from low-active estrone (E1), is over-expressed in endometrial cancer (EC) thus implicating an increased intra-tissue generation of E2 in this estrogen-dependent condition. Objective: Explore the possibility to inhibit 17βHSD-1 and impair the generation of E2 from E1 in EC for potential therapeutic purposes. Various in vitro, in vivo and ex vivo models were used. Methods and Patients: We generated EC cell lines derived from the well-differentiated endometrial adenocarcinoma Ishikawa cells and expressing levels of 17βHSD-1 similar to human tissues (Ishi-HSD1). High-performance-liquid-chromatography (HPLC) was used to measure the 17βHSD-1 activity in cell free assay. Estrogen dependent growth and the ability to inhibit 17βHSD-1 activity were assessed in a colony formation assay and in vivo using the chicken chorioallantoic membrane assay (CAM). Two retrospective EC patient cohorts were used to test inhibition (HPLC) and to determine 17βHSD-1 expression (RT-PCR) in paired primary/metastatic lesions. Results: Using Ishi-HSD1 cells, E1 to E2 conversion and 17βHSD-1 activity were blocked by a specific 17βHSD-1 inhibitor in cell-free assay followed by HPLC analysis. In vitro, E1 administration elicited colony formation similar to E2, and this was impaired by 17βHSD-1 inhibition. In vivo, tumours grafted on the CAM demonstrated that E1 upregulated the expression of the estrogen responsive cyclin A similar to E2, which was impaired by 17βHSD-1 inhibition. Neither in vitro nor in vivo effects of E1 were observed using 17βHSD-1 negative cells (negative control). Using a patient cohort of 52 primary ECs, we demonstrated the presence of 17βHSD-1 enzyme activity, which was inhibited using the 17βHSD-1 inhibitor by over 90% in more than 45% of ECs. Since drug treatment is generally indicated for metastatic/recurrent and not primary tumour, we next demonstrated the mRNA expression of the potential drug target, 17βHSD-1, in metastatic lesions using a second cohort of 37 EC patients. Conclusion: 17βHSD-1 inhibition efficiently blocks the generation of E2 from E1 using various EC models. Further preclinical investigations and 17βHSD-1 inhibitor development to make candidate compounds suitable for the first human studies are awaited.