Mycosis fungoides is an epidermotropic T-cell lymphoma that primarily affects the skin, with a widely variable clinical presentation that frequently demonstrates a non-aggressive, slowly growing cutaneous lesion in the form of a pruritic and erythematous flat patch, whose diagnosis can be extremely difficult due to strong similarities with other reactive conditions. Sezary syndrome represents a leukemic disease associated with a more aggressive behavior and poorer prognosis that is characterized by erythroderma, generalized lymphadenopathy and circulating tumor cells. Male patients are the most affected, usually among the elderly population. Oral manifestations in both disorders are very uncommon, but well-documented cases are available in the literature. In the large majority of cases, cutaneous lesions precede the diagnosis of oral lesions. Multiple oral lesions are more frequently diagnosed, and the tongue, palate and gingiva are the most affected locations. The clinical presentation varies significantly from ulcers to erythematous plaques to nodules or tumors. Microscopically, these diseases are characterized by a diffuse infiltrate of atypical small to medium-sized T lymphocytes. Epidermotropism is a very common finding and neoplastic cells present cleaved nuclei, leading to the so-called “cerebriform” cells. T-cell markers may be lost and the diagnosis of oral lesions is usually very challenging.