Oral leukoplakia is a potentially malignant disorder affecting the oral mucosa. It is defined as "essentially an oral mucosal white lesion that cannot be considered as any other definable lesion." Oral leukoplakia is a white patch or plaque that develops in the oral cavity and is strongly associated with smoking. Leukoplakia is a firmly attached white patch on a mucous membrane which is associated with increased risk of cancer. The edges of the lesion are typically abrupt and the lesion changes with time. Advanced forms may develop red patches. There are generally no other symptoms. It usually occurs within the mouth, although sometimes mucosa in other parts of the gastrointestinal tract, urinary tract, or genitals may be affected.

The cause of leukoplakia is unknown. Risk factors for formation inside the mouth include smoking, chewing tobacco, excessive alcohol, and use of betel nuts. One specific type is common in HIV/AIDS. It is a precancerous lesion, a tissue alteration in which cancer is more likely to develop. The chance of cancer formation depends on the type, with between 3–15% of localized leukoplakia and 70–100% of proliferative leukoplakia developing into squamous cell carcinoma.

Leukoplakia is a descriptive term that should only be applied after other possible causes are ruled out. Tissue biopsy generally shows increased keratin build up with or without abnormal cells, but is not diagnostic. Other conditions that can appear similar include yeast infections, lichen planus, and keratosis due to repeated minor trauma. The lesions from a yeast infection can typically be rubbed off while those of leukoplakia cannot.

Treatment recommendations depend on features of the lesion. If abnormal cells are present or the lesion is small surgical removal is often recommended; otherwise close follow up at three to six month intervals may be sufficient. People are generally advised to stop smoking and limit the drinking of alcohol. In potentially half of cases leukoplakia will shrink with stopping smoking; however, if smoking is continued up to 66% of cases will become more white and thick. The percentage of people affected is estimated at 1–3%. Leukoplakia becomes more common with age, typically not occurring until after 30. Rates may be as high as 8% in men over the age of 70.