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Lichen sclerosus (LS) and lichen planus (LP) are two conditions frequently affecting genital skin whose clinical and histologic distinction can be difficult. Both diseases can feature solitary genital lesions with bandlike lymphocytic infiltrates. We reviewed 68 cases of vulvar LS to find sections that contained a transition from a lichenoid interface reaction to pathognomonic LS (i.e., marked papillary dermal sclerosis or edema), and in these nine cases we studied routinely and specially stained sections, as well as sections stained with a panel of antisera to lymphoid antigens, and compared the findings with those in six cases of genital LP. We assumed that changes at the periphery of a lesion of LS mirror findings seen in early lesions. The features that we found more commonly in the inflammatory phase of LS included a psoriasiform lichenoid pattern (100% LS, 0% LP), basilar epidermotropism (78% LS, 0% LP), loss of papillary dermal elastic fibers (100% LS, 33% LP), basement membrane thickening (44% LS, 0% LP), and epidermal atrophy (33% LS, 0% LP). Features found more commonly in LP included many cytoid bodies (0% LS, 100% LP), wedge-shaped hypergranulosis (11% LS, 100% LP), basal squamatization (22% LS, 100% LP), and pointed rete ridges (11% LS, 83% LP). We did not detect any significant differences in the immunohistochemical features of the infiltrates. Taken together, these histologic features comprise light microscopic criteria for the diagnosis of early vulvar LS and its differentiation from LP.