A painful, 4.8-cm nodule developed over 3 weeks on the face of a 76-year-old man. The results of a generous shave biopsy suggested a keratoacanthoma, a benign but locally destructive tumor that typically arises abruptly and enlarges rapidly. Curettage and desiccation were subsequently performed. However, writes Dr Ted Rosen of Houston, the lesion promptly recurred. It is highly unusual for a keratoacanthoma to recur after removal; this raised the suspicion that the lesion was actually a squamous cell carcinoma. Both intralesional injections of 5-fluorouracil and oral isotretinoin were tried but to no avail. The entire lesion—which now measured 6 cm in diameter—was excised; pathologic examination revealed invasive, well-differentiated squamous cell carcinoma. The patient was given postoperative local radiotherapy. Dr Rosen points out that this tumor illustrates the potential difficulty in distinguishing keratoacanthoma from squamous cell carcinoma clinically as well as histologically.