Glucose transporter type I (GLUT1), a prototype member of GLUT super-family, reacts with a 55 kDa protein and is a membrane-associated erythrocyte glucose transport protein. It is a major glucose transporter in the mammalian blood-brain barrier and mediates glucose transport in endothelial cells of the vasculature, adipose tissue, and cardiac muscle. GLUT1 is detectable in many human tissues including those of colon, lung, stomach, esophagus, and breast. GLUT1 is overexpressed in malignant cells and in a variety of tumors that include the breast, pancreas, cervix, endometrium, lung, mesothelium, colon, bladder, thyroid, bone, soft tissues, and oral cavity. Immunohistochemical detection of GLUT1 can discriminate between reactive mesothelium and malignant mesothelioma according to some observers. However, more recent reports have questioned this application. Anti-GLUT1 is a useful marker that can be applied to cytologic and histologic specimens. It can be used as a reliable component of an antibody panel to distinguish reactive mesothelial cells from metastatic adenocarcinoma particularly adenocarcinomas of body cavity effusions as well as in certain adenocarcinomas of ovarian and pulmonary origin. Distinguishing adenocarcinoma from hepatocellular carcinoma (HCC) in liver tumors can be quite challenging. Anti-GLUT1, in combination with antibodies against CA 15-3 and Hep Par1, is quite helpful in the discrimination of HCC from other carcinomas.