Mycosis fungoides (MF) is the most common primary cutaneous epidermotropic T-cell lymphoma (80%). The accurate diagnosis of MF confirmed only by clinical, histological and immunohistochemical signs amounts to 50—75%. Objective — to investigate genetic markers (FOXP3, STAT4, IL-12B) for the early diagnosis of MF, to estimate the informative value of used diagnostic techniques (histology, immunophenotyping), and to determine clonality by the T-cell receptor γ-chain genes. Subject and methods. Fifty patients with MF and plaque parapsoriasis (PP) who had been treated at the V.A. Rakhmanov Clinic of Skin and Venereal Diseases and at the National Medical Research Center for Hematology were followed up. A MF group consisted of 27 patients; a PP group included 23 patients, and a control group comprised 10 healthy individuals. The expression of the FOXP3, STAT4, and IL-12B genes was analyzed by TaqMan real time-PCR. The objectives of the study were affected skin portions from patients with MF or PP and healthy individuals. Results. The investigation revealed a increase in the expression level of STAT4 mRNA transcripts by 9 times in patients with MF compared with those with PP and by 553 times in healthy individuals. There was also a statistically significant predominance of the expression level of STAT4 mRNA transcripts in patients with spotted and plaque stages of MF (180; 318) compared with those with PP and healthy individuals, as well as a sharp decrease in those with erythrodermic MF, which was statistically significant. Conclusion. MF cannot be diagnosed without comprehensively assessing the clinical, anamnestic, histological, immunophenotypic, and molecular genetic data. The expression level of STAT4 mRNA transcripts is of great importance for the early diagnosis of MF. Inclusion of the level of STAT4 expression in the list of diagnostic signs increases the accuracy of differential diagnosis of MF and PP from 59.1 to 81.8%, respectively.