The histologic differential diagnosis of perianal Paget disease includes malignant melanoma, pagetoid spread of squamous cell carcinoma, and secondary involvement by colorectal carcinoma. While consideration of these entities is useful when establishing a diagnosis, it does not apply when patients with Paget disease undergo surveillance for recurrent disease. Treatment of perianal Paget disease consists of a combination of surgical excision with skin grafts and topical chemotherapeutic agents that induce cytologic alterations in benign cells and simulate recurrent malignancy. To evaluate the therapy-related changes and possible diagnostic pitfalls in patients with Paget disease, we reviewed 412 posttreatment tissue samples from 3 women with primary perianal Paget disease who underwent wide excision, skin grafting, and topical 5-fluorouracil therapy. Biopsy samples from engrafted skin often displayed single and clustered cells with hyperchromatic nuclei dispersed in the deep epidermis. Similar cells were scattered throughout all levels of the epidermis in biopsy samples following topical chemotherapy. The abnormal cells were negative for cytokeratin 7 (CK7) and mucicarmine in both situations. Disease ultimately recurred in all patients; some Paget cells showed classic features with eosinophilic or mucinous cytoplasm and eccentric nuclei, whereas others were smaller with less conspicuous atypia. All Paget cells showed strong, membranous CK7 staining. In short, treatment of perianal Paget disease can elicit cytologic abnormalities in benign epithelial cells that simulate the cytologic features of recurrent disease, and can diminish the atypia of Paget cells. Immunohistochemical stains for CK7 can be helpful when evaluating surveillance samples from these patients.