Overview Mycosis fungoides (MF), formerly known as mycosis fungoides, is a type of cutaneous T-cell lymphoma. It is the most common type of primary cutaneous T-cell lymphoma, accounting for 54% of all cutaneous lymphomas. It is characterized by the final development of a neoplastic stage after a patchy or plaque stage, or other clinicopathological variants with a similar clinical course [1]. Epidemiology Mycosis fungoides is a rare disease with an annual incidence of 0.3 cases per 100,000 people. It occurs mainly in middle-aged and elderly people, with an average age of onset of 55-60 years, but it can also occur in children and adolescents. It is more common in men than in women, with a male to female incidence ratio of 1.6-2.0:1 [2]. Pathogenesis The etiology and pathogenesis of this disease are unknown, but may be related to cytogenetic, immune, and environmental factors. Clinical presentation Classic mycosis fungoides presents clinically with three stages of lesions, namely the erythematous, plaque, and tumor stages, but the three stages may partially overlap, and thus all three stages may be seen together clinically. Mycosis fungoides progresses slowly, often over several years or even decades. Early manifestations are nonspecific eczema-like or psoriasis-like lesions that often require multiple skin biopsies before diagnosis is confirmed. The average time from the onset of lesions to diagnosis is 4-6 years [3]. (i) Erythematous lesions are mostly erythematous patches of variable size with mild desquamation, usually well-defined and ranging in color from orange-red to dark purplish-red. Sometimes there are varying degrees of atrophy and heterochromatic skin changes, such as punctate hyperpigmentation or hypopigmentation, with capillary dilation. Early lesions tend to occur on the buttocks and non-exposed parts of the trunk and extremities; itching or no obvious conscious symptoms are observed, and occasionally the lesions fade without leaving a scar. It is clinically similar to psoriasis, eczema, chronic contact dermatitis, atopic dermatitis, neurodermatitis, etc. and is easily misdiagnosed. Therefore, multiple skin biopsies are usually required to clarify the diagnosis. Most of the large plaques of parapsoriasis en plaques evolve into mycosis fungoides over time.