Thorough examination may lead to the detection of extraoral manifestations of LP that provide additional support for the diagnosis or the identification of clinical findings that suggest another diagnosis. Tissue biopsies of oral LP help to confirm the diagnosis and are particularly of value for erythematous and erosive LP, which share features with multiple other mucosal disorders, including oral malignancy. Biopsies to confirm oral LP are less essential in patients who present with classic reticular LP, particularly in patients in whom a diagnosis of LP has already been confirmed through biopsy of an extraoral manifestation of this disorder. Differential diagnosis Skin Lichenoid drug eruption The cutaneous manifestations resemble idiopathic lichen planus. Chronic graft-versus-host disease The history of preceding hematopoietic cell transplant is helpful for diagnosis Psoriasis Atopic dermatitis Cutaneous lupus erythematosus Discoid lupus erythematosus Mouth Oral lichenoid drug reaction Lichenoid drug eruptions may be caused by a variety of systemic medications and share clinical features with oral LP. Histologic findings of a deep mixed infiltrate with lymphocytes, plasma cells, and neutrophils (with or without eosinophils) and perivascular inflammation favor this diagnosis. Oral lichenoid contact reaction (allergic contact mucositis) Oral lichenoid contact reactions may be caused by a variety of substances. The clinical and histologic features of oral lichenoid contact reactions are similar to oral LP. Patch testing and recognition of the proximity of an offending substance to the eruption can aid with diagnosis. Autoimmune blistering diseases Mucous membrane pemphigoid and other autoimmune blistering diseases may present with oral erosions and desquamative gingivitis similar to that seen in erosive LP. Biopsies for routine histologic examination and direct immunofluorescence are useful for distinguishing these disorders from oral LP. Graft-versus-host disease (GVHD) Lacy, reticulated plaques or erosions that resemble oral LP may occur in GVHD. The histologic findings of these disorders are also similar. The patient history is useful for differentiating chronic GVHD from oral LP. Oral involvement in acute GVHD is less well characterized than chronic GVHD, but has been associated with erythematous, erosive, ulcerative, or lichenoid oral lesions. Leukoplakia Leukoplakia is a manifestation of squamous epithelial hyperplasia that may be a precursor to oral squamous cell carcinoma. White patches or plaques usually appear on the oral mucosa. To rule out malignancy, a biopsy of leukoplakia is indicated. Oral squamous cell carcinoma Oral squamous cell carcinoma (SCC) can present as erythematous or white patches, ulcers, or exophytic masses. The highest risk for