Female CF patients suffer from sub-to infertility, often facing problems to become pregnant. Underlying reasons remain understudied. In particular, it is largely unknown whether dysfunction of the endometrium is involved, the womb’s inner lining and key tissue for embryo implantation and development. This gap is mainly due to lack of appropriate study models. Therefore, we develop and apply tissue-mimicking organoid models to indetail decipher molecular and functional aberrations in fertility-deficient CF endometrium as compared to healthy (fertile) endometrium. First, we established endometrial organoids (EMO) from the Cftrtm1Eur mouse model. These organoids show a smaller lumen than wildtype (WT) EMO, and do not swell in the FIS assay, both validating the CFTR defect. Currently, we are comparing expression of endometrial functionality/fertility markers in CF versus WT EMO, as well as their responsiveness to estrogen (E2) and progesterone (P4), which in vivo regulate the estrous cycle. Second, we develop(ed) organoids from CF patient endometrium. Before, we have shown that EMO from healthy endometrium can reliably reproduce all menstrual cycle phases under defined E2/P4 exposure. Currently, we are deciphering whether and how menstrual cycle phases, including the receptivity stage, are different between CF and healthy EMO. We already observed dissimilarities in the proliferative (E2) and secretory (P4) phase, such as increased apoptosis and decreased fertility marker expression, respectively. Taken together, we establish(ed) organoid models from CF endometrium as novel and powerful tools to gain insight into the endometrial factor in CF fertility deficiency. Importantly, the organoids will be highly apt to explore the impact of new-era CFTR modulators on the endometrium, at present unknown. Moreover, our study has the potential to reveal paths toward restoring reproductive fitness in CF patients, which can be tested using the organoids as (drug) screening platform.