Thirty cases of poorly differentiated carcinomas of the skin were examined for the expression of vimentin. All cases expressed cytokeratins; in addition, 12 cases were positive for vimentin. These were all non-reactive with antibodies to S100 protein, HMB45 and desmin. The finding of vimentin in poorly differentiated squamous cell carcinomas underscores the need for caution in the use of immunohistochemical stains for tumor typing. Cutaneous squamous cell carcinomas are an addition to the list of epithelial tumors which are known to coexpress vimentin intermediate filaments. Other carcinomas which consistently express vimentin include those of renal, endometrial, thyroid, pulmonary, ovarian, salivary gland, adrenal and more recently, those of breast and prostatic orgin.