Mycosis fungoides (MF) and Sézary syndrome (SS) belong to the group of primary cutaneous T-cell lymphomas (CTCL). Regardless of the stage of the disease, patients with MF and SS can suffer from chronic pruritus. The aim of the study was to investigate the correlation between the interleukin 31 (IL-31) serum level, the degree of pruritus and CTCL severity; and to compare the frequency of IL-31 gene polymorphisms between patients and the control group, and between patients at different stages of the disease. Pruritus affected 67.7% of patients with MF and SS in our study. The IL-31 serum level was significantly higher in CTCL patients than in the control group but there were no positive correlation between IL-31 serum level and pruritus. A statistically significant difference in allele frequencies for IL-31 IVS2+12 gene polymorphisms between early and advanced stages was detected; GAG haplotype was more frequent and AGA was less frequent in stage IA patients compared with patients in the other stages of the disease.