Differentiated vulvar intraepithelial neoplasia (dVIN), precursor of vulvar squamous cell carcinoma, is human papilloma virus independent and often found in a background of lichen sclerosus (LS) and lichen simplex chronicus (LSC). Subtle histologic findings make the diagnosis of dVIN difficult, and, although the use of p53 and Ki-67 has been of some value, there is a need for a better immunohistochemical marker. Cytokeratin 17 (CK17), a cytoskeletal intermediate filament protein, has previously been used in the diagnosis of anogenital lesions. Here we evaluated CK17 in dVIN in comparison with LS, LSC, and usual VIN (uVIN/HSIL). Twenty-nine cases of dVIN, 9 cases of uVIN, 8 cases of LS, and 7 of LSC were evaluated using CK17, Ki-67, and p53. All 29 dVIN cases displayed immunoreactivity for CK17, with 27 (93%) showing intermediate to strong and diffuse reactivity. No cases of uVIN displayed diffuse CK17 expression, whereas 63% of LS and 29% of LSC displayed intermediate to strong diffuse immunoreactivity, confined to the upper half of the epithelium. P53 and Ki-67 expression was present in varying degrees in all types of lesions, displaying limited discriminatory power for dVIN. Our findings suggest that CK17, although not specific for dVIN, when combined with histologic findings, Ki-67, and p53 immunohistochemistry, can be a marker of vulvar dysplasia and serve as an adjunct in the diagnosis of dVIN. Specifically, in small biopsies, the presence of diffuse suprabasal or full thickness expression strongly favors a diagnosis of dVIN over LSC, whereas focal and/or superficial expression supports a diagnosis of LSC.