Background. Mycosis fungoides (MF) is the most common disease among the cutaneous T-cell lymphomas (85-90%). The accuracy of the diagnosis of MF, which is confirmed only by clinical, histological and immunohistochemical signs, is 50-75%. The aim of the study was to investigate genetic markers (FOXP3, STAT4, IL-12B) for early diagnosis of mycosis fungoides. Material and methods. A study involving 42 patients with MF and plaque parapsoriasis (PP) treated at the Dermatology Department of I.M. Sechenov First Moscow State Medical University and National Medical Hematology Research Сenter, was performed. The analysis of gene expression FOXP3, STAT4, IL-12B was carried out by TaqMan Real time-PCR. The objects of the study were lesional skin samples of patients. A group with MF consisted of 29 patients, a group with PP consisted of 13 patients, a control group included 10 healthy volunteers. Results. The study revealed that the level of STAT4 gene expression showed a significant (9 times) increase in the mRNA expression of STAT4 transcripts in patients with MF (166) compared with patients with PP (17.9; p < 0.05) and 553 times - with healthy volunteers (0.3; p < 0.05). There was also a statistically significant predominance of the level of mRNA expression of STAT4 transcripts in patients with spotted and plaque stages of MF (180; 318) compared with patients with PP (17.9; p < 0.05) and healthy volunteers (0.3; p < 0.05), as well as a sharp decrease in patients with erythrodermic form of MF (7.19). For early diagnosis of MF the level of expression of mRNA transcripts STAT4 is of great importance. Inclusion of STAT4 in the list of diagnostic features increases the accuracy of differential diagnosis of MF and PP from 59.1 to 81.8%.