Oral mucous membrane lesions are often clues to the presence of systemic illnesses, such as acquired immunodeficiency syndrome (AIDS), syphilis, and systemic lupus erythematosus. They may also be helpful in diagnosing dermatologic conditions, including lichen planus and pemphigus vulgaris. “Canker sores” (aphthous ulcers) are often seen as isolated phenomena, but may also be an accompaniment or a precursor to a systemic disease such as Behçet disease and ulcerative colitis.

Mucous membrane lesions most frequently appear as erosions, blisters, fissures, ulcers, whitish plaques, pigmentary changes, neoplasms, cystic lesions, or as normal variants. It is often difficult to make a clinical diagnosis in the oral cavity. On the mucous membranes, leukoplakia, squamous cell carcinoma, and warts may appear indistinguishable from one another. Inflammatory oral lesions, particularly those that are vesicobullous, rarely remain intact and unruptured; instead, such lesions usually become erosions and ulcers by the time clinicians see them, adding to the difficulty of diagnosis.