Therefore, the aim of this study was to explore the presence and role of activated fibroblasts in the microenvironment of endometriosis with regard to FAP expression, stroma composition, and immune cell infiltrate in order to broaden the understanding of the disease and to explore further diagnostic options such as FAPi PET/CT targeted at activated fibroblasts in the stroma. The gene discussed is FAP; the disease is endometriosis.