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Follistatin is a potent native activin antagonist that is expressed in the normal mammary gland and in different breast proliferative diseases. Despite experimental evidence that follistatin can modulate the breast cancer cell cycle, the clinical significance of follistatin expression in these tumors is unknown. The aim of this study was to correlate the intensity of follistatin expression in invasive breast cancer with some of its clinical and pathologic features, such as the disease stage and the hormonal receptor status. Paraffin blocks of tumor samples that had been fixed in buffered formalin were obtained from 154 women subjected to surgery for breast cancer between 2008 and 2012. Sections from all paraffin blocks were cut and processed together by immunohistochemistry using a commercial monoclonal antibody to human follistatin. The intensity of follistatin staining was unrelated to the menopausal status, the disease stage, the grade, progesterone receptor expression, and local or systemic recurrence. However, follistatin immunoreactivity was significantly stronger in estrogen receptor (ER)-negative tumors than in ER-positive tumors. These findings suggest that follistatin expression in invasive breast cancer is unrelated to the disease severity and the risk of recurrence, but is more intense in ER-negative tumors.