Background: The increased glucose uptake seen in cancer cells correlates with the expression of human erythrocyte glucose transporter (Glut1) protein in certain human malignancies. Objective: Our purpose was to determine Glut1 expression in cutaneous neoplasms. Methods: A polyclonal anti-Glut1 antibody (MYM) and a standard ABC immunoperoxidase technique were used to determine Glut1 expression in invasive squamous cell carcinomas (SCCs), SCC in situ, basal cell carcinomas (BCCs), melanomas, actinic keratoses (AKs), seborrheic keratoses, common acquired nevi, and scars with regenerative epidermal hyperplasia. Results: All of the cases of SCC in situ, 14 of 15 (93%) of the SCC, and 13 of 15 AKs (87%) showed intense membranous staining for Glut1. Glut1 staining was present in the epidermis of 8 of 15 scars (53%) but was not detected in any BCC, even in areas of focal keratinization and squamous metaplasia. Glut1 reactivity was absent in the melanomas and seborrheic keratoses. Conclusion: Glut1 expression in a cutaneous lesion strongly suggests a proliferative lesion of the squamous cell type. (J Am Acad Dermatol 1997;37:575-7.)