The inclusion of elastic fibers within the epithelium of keratoacanthomas is a phenomenon suggested to be an aid in differentiating this lesion from squamous cell carcinoma. Antilysozyme antibodies have recently been noted to stain actinically damaged elastic fibers but not those from sunprotected skin. In this study, 54 keratoacanthomas and 46 squamous cell carcinomas were stained with a histochemical elastic tissue stain and polyclonal antibody to lysozyme using an immunoperoxidase technique. Elastic fibers were demonstrated in keratoacanthomas (37/54, 68%) significantly more often than squamous cell carcinomas (12/46, 26%) (p < 0.001) using both techniques. This study confirms that the inclusion of elastic fibers occurs significantly more often in keratoacanthomas than squamous cell carcinomas. These elastic fibers were also actinically damaged, suggesting a role for sun damage in the evolution of keratoacanthoma.