A dome-shaped, rapidly growing skin lesion composed of well differentiated squamous cells. It represents a proliferation of the infundibular epithelium of the hair follicle and its morphologic distinction from a well differentiated carcinoma may be difficult or impossible. Keratoacanthomas affect males more frequently than females and the majority tend to regress spontaneously. It has been suggested that keratoacanthoma represents a distinct subtype of squamous cell carcinoma of the skin. [ ]