Thoracic endometriosis (TE) is rare with positive histological diagnosis sometimes difficult, particularly in atypical form. The aim of this study was to identify features which can increase performance of the histolological TE diagnosis and more particularly immuno-histochemical (IHC) contribution with hormonal receptors, smooth muscle actin, Ber-Ep4, CD10 and calretinin antibodies. To address this issue, we retrieved, retrospectively, a large series of 591 pneumothorax operated. Among them, 135 (23%) were females including eight (6%) cases related to TE. Those eight women were surgically treated with resection of pleura (n=6/8), lung (n=5/8) and diaphragmatic samples (n=6/8). Typical histological lesions of endometriosis were observed in six cases among eight. All diaphragmatic samples presented, macroscopically, holes responsible of thoraco-abdominal communication (n=6/6). Endometrial glands and/or endometrial stroma cells were found in the diaphragm (n=5/6) and in the pleura (n=2/6) but were never encountered in the lung (n=0/5). IHC study can confirm the five diaphragmatic localizations and can identify a new localization with expression of hormonal receptors, CD10 and smooth muscle actin in an island of fusiform cells. In conclusion, our study shows 1) the high frequency of diaphragmatic endometriosis localization which holes existence also can explain the pathogenesis, 2) the value of diaphragmatic samples in positive histological diagnosis of TE, 3) IHC interest to confirm endometriosis, particularly in atypical form and to differentiate from mesothelial cells inclusion.