Mycosis fungoides is the most common form of primary cutaneous lymphoma, with three phases: erythematous, infiltrative and tumour phase. In contrast with Sézary syndrome, the course is often slow and skin lesions may not be distinctive for a longer period of time and may resemble lesions occurring in the course of other dermatoses, such as eczema, large plaque parapsoriasis, atopic dermatitis or psoriasis. The right choice of treatment method seems to be essential in the therapy of mycosis fungoides. According to the consensus by the Cutaneous Lymphoma Section of the Polish Lymphoma Research Group, less aggressive methods of treatment mainly with local administration of corticosteroids and phototherapy should be used in early stages of the disease. The described case is a patient with mycosis fungoides, in whom modern UVA1 therapy proved to be successful. The obtained remission was confirmed by ultrasound imaging of skin lesions before and after the therapy, which may be a new method of monitoring the course of the disease in patients with mycosis fungoides.