Pepsinogen C is a proteolytic enzyme involved in the digestion of proteins in the stomach; it is also synthesized by a significant percentage of female breast carcinomas. In addition, it has been demonstrated that pepsinogen C is one of the few proteins induced by androgens in breast carcinoma cells. Here we evaluate the expression of pepsinogen C by immunoperoxidase staining in normal breast tissue from 3 male patients, 15 gynecomastia tissues, 2 male in situ breast carcinomas, and 68 male invasive breast carcinomas. Pepsinogen C immunostaining values were quantified in male breast tumors using the HSCORE system, which considers both the intensity and the percentage of cells staining at each intensity. The results indicated positive immunohistochemical staining for pepsinogen C in all gynecomastia tissues, the two in situ ductal carcinomas, and 52 of 68 invasive breast carcinomas (76.4%). The three normal breast tissues analyzed showed negative staining for pepsinogen C, whereas invasive tumors showed clear differences among them with regard to the intensity and percentage of staining cells. In addition, pepsinogen C scores were significantly higher in well-differentiated (grade I, 188.7) and moderately differentiated (grade II, 145.8) tumors than in poorly differentiated (grade III, 98.5) tumors (p = 0. 032). Similarly, significant differences in pepsinogen C content were found between estrogen receptor (ER)-positive tumors and ER-negative tumors (158.5 vs. 44.3, respectively; p = 0.009). Patients with pepsinogen C-positive tumors reached longer relapse-free and overall survival periods than did those with tumors with negative staining, but no statistical differences were observed between survival curves calculated for these two groups of patients. This results demonstrate expression of pepsinogen C by gynecomastias and by a high percentage of male breast carcinomas and may indicate an important role of pepsinogen C in the pathophysiology of male breast diseases.