To evaluate the immunohistochemical expression of GLUT1, human erythrocyte glucose transporter 1, and fatty acid synthase (FAS), 66 human breast carcinomas and adjacent peritumoral tissue were studied. GLUT1 and FAS were expressed in 53 and 61 carcinomas, in 17 and 14 typical/atypical hyperplastic tissues, and in 16 and 13 tissues adjacent to tumor normal breast tissue, respectively. Statistical analysis revealed association between invasive carcinomas, invasive carcinomas with in situ component and GLUT1 immunostaining. GLUT1 staining was associated with tumor grade, FAS with tumor stage, and GLUT1 and FAS coexpression with tumor grade. Controls expressed no immunostaining. GLUT1 and FAS are new markers involved in the biologic activities of cancer cells. GLUT1 and FAS coexpression may indicate increased use of energy by the neoplastic cells correlated with poorly differentiated features and aggressive behavior. The innovative finding that GLUT1 and FAS are observed in mammary carcinoma adjacent nonneoplastic tissues may suggest a role in detecting initial phases of breast carcinogenesis.