Objective:To investigate molecules involved in the occurrence of pruritus in patients with mycosis fungoides (MF) .Methods:Totally, 522 patients with MF were enrolled from Peking University First Hospital from October 2009 to August 2021, and the incidence of pruritus was calculated. The patients were grouped according to whether they suffered from pruritus or not. RNA sequencing data on biopsied skin lesions of 49 patients were analyzed to identify differentially expressed genes between patients with pruritus and those without; enzyme-linked immunosorbent assay and immunohistochemical techniques were performed to determine the protein expression of CC chemokine ligand 17 (CCL17) in serum samples from 88 MF patients, and in tissue samples from 81 MF patients, respectively; flow cytometry was conducted to detect markers for T lymphocyte activation and differentiation in peripheral blood samples from 46 MF patients to identify peripheral blood lymphocyte subsets associated with pruritus. Statistical analysis was carried out by using chi-square test, Mann-Whitney U test, and Spearman correlation analysis. Results:Among the 522 patients with MF, 305 were males and 217 were females; 347 were diagnosed with early-stage MF, and 175 with advanced MF. The incidence of pruritus was 67.2% (351/522) in the patients with MF, and significantly higher in the patients with advanced MF (81.7%, 143/175) than in those with early-stage MF (59.9%, 208/347; χ2 = 25.03, P < 0.001) . RNA sequencing showed that CCL17 mRNA expression was significantly higher in the MF patients with pruritus than in those without (fold change = 10.09, P < 0.001) . The serum CCL17 concentration was significantly elevated in the patients with pruritus (1 017.05[377.12, 4 831.80] pg/ml) compared with those without (361.66 [180.47, 500.08] pg/ml; Z = -4.57, P < 0.001) , and correlated with pruritus scores ( r = 0.57, P = 0.010) . In both early and advanced stages of MF, the serum CCL17 concentration was significantly higher in the patients with pruritus than in those without ( Z = -3.68, P < 0.001; Z = -2.54, P = 0.011, respectively) . Immunohistochemical staining revealed that there was no significant difference in the relative quantification value of CCL17 between the patients with pruritus and those without ( Z = -1.84, P = 0.066) . The percentage of CD3 +CD4 +CD26 -CCR4 + malignant T cells significantly increased in the MF patients with pruritus than in those without ( Z = -2.03, P = 0.043) , and was positively correlated with serum CCL17 concentrations ( r = 0.49, P < 0.001) . Conclusions:Both CCL17 mRNA expression in lesional tissues and serum CCL17 concentrations increased in MF patients with pruritus, and CCL17 was associated with the occurrence of pruritus. CCL17 may be involved in the occurrence of pruritus through the recruitment of CD3 +CD4 +CD26 -CCR4 + malignant T cells.