Mycosis fungoides, also known as Alibert-Bazin syndrome or granuloma fungoides, is the most common form of cutaneous T-cell lymphoma. It generally affects the skin, but may progress internally over time. Symptoms include rash, tumors, skin lesions, and itchy skin.

While the cause remains unclear, most cases are not hereditary. Most cases are in people over 20 years of age, and it is more common in men than women. Treatment options include sunlight exposure, ultraviolet light, topical corticosteroids, chemotherapy, and radiotherapy.

The symptoms of mycosis fungoides are categorized into three clinical stages: the patch stage, the plaque stage, and the tumour stage. The patch stage is defined by flat, reddish patches of varying sizes that may have a wrinkled appearance. They can also look yellowish in people with darker skin. The plaque stage follows the patch stage of mycosis fungoides. It is characterized by the presence of raised lesions that appear reddish-brown; in darker skin tones, plaques may have a greyish or silver appearance. Both patch and plaque stages are considered early-stage mycosis fungoides. The tumour stage typically shows large irregular lumps. Tumours can develop from plaques or normal skin in any region of the body, including the face and head regions.

The symptoms displayed are progressive, with early stages consisting of lesions presented as scaly patches. Lesions often initially develop on the trunk of the body in places that are rarely exposed to the sun, such as the buttocks. These lesions can start as insignificant patches and may remain undiagnosed for up to a decade. Hypopigmentation (when the skin is lighter than normal) of lesions are less common but can be found in children, adolescents and/or dark-skinned individuals.

The advanced stage of mycosis fungoides is characterized by generalized erythroderma (red rash covering most of the body) with severe pruritus (itching) and scaling. Itching (pruritus) is the most commonly reported symptom of people experiencing mycosis fungoides with up to 88% of people reporting varying intensities of pruritus that typically worsens as the disease progresses. Those that experience intense pruritus commonly indicate that it negatively affects their quality of life emotionally, functionally and physically.

Mycosis fungoides (MF) and Sézary syndrome (SS) are related conditions, with the same type of cancer T-lymphocytes, that initially grow in different body compartments. SS cells are found mainly in the blood, whereas MF typically involves the skin. In advanced stages of MF, the cancer cells move from the skin into other organs and the bloodstream; this progression is referred to as "leukemic mycosis fungoides", "Sézary syndrome preceded by mycosis fungoides", or "secondary mycosis fungoides".

Mycosis fungoides is caused by abnormal white blood cells (T-lymphocytes). These abnormal cells have a preference for localizing and proliferating uncontrolled in the outer layer of the skin (epidermis). The abnormal cells may later involve other organs such as the lymph nodes. It is hypothesized that the genetic mutations in these cancer cells lead to increased growth and escape from programmed cell death.

Additionally, the disease is an unusual expression of CD4 T cells, a part of the immune system. These T cells are skin-associated, meaning they are biochemically and biologically most related to the skin, in a dynamic manner. Mycosis fungoides is the most common type of cutaneous T-cell lymphoma (CTCL), but there are many other types of CTCL that have nothing to do with mycosis fungoides and these disorders are treated differently.