The focus of the presented work was to isolate and characterize circulating endometrial cells (CECs) enriched from peripheral blood (PB) of patients with diagnosed endometriosis to support endometriosis diagnosis and treatment. Material and Methods Blood samples (n=423) were tested for CECs presence. Subsequently, gene expression analysis (GEA) was carried out for CECs. In parallel, the presence and character of CECs was tested during phases of menstrual cycle (MC) (n= 11). CECs were isolated by size-based separation from 2x 8ml PB. Results CECs were detected in 78.4% of blood samples. In line with the revised American Fertility Society (rAFS) classification CECs presence was confirmed in all the acknowledged endometriosis stages: minimal, mild, moderate and severe. Surprisingly, the highest CEC negativity rate was reported for severe disease (21.1%). The highest CEC numbers were detected in the mid-secretory periods of MC, which corresponds with uterine lining decidualization. The cytomorphology of CECs captured during MC is changing between the epithelial, stromal and stem cell-like. CECs captured in mid-secretory period expressed KRT18, NANOG and VIM in higher amounts when compared to the genes in the proliferative phase of MC when KRT19 and ESR1 were mostly observed. GEA of the super-positive CECs samples (1000 CECs/8mL PB) revealed high expression of KRT18, VIM, NANOG and FLT1. The expression of these genes was elevated in the endometriosis tissue samples and endometrioma, too. Conclusion The panel of the identified CECs - genes could be tested in a prospective manner to confirm the predictive value of CECs in endometriosis diagnostics.