The aim of our study was to investigate if the Treg subsets and STAT5 phosphorylation (pSTAT5) are altered in CD4+ T cells in the blood of patients with CLL and whether the changes in circulating Treg subsets are related to STAT5 signalling imbalances, to the Binet stage, to the estimate of the size of tumour mass and to the disease course during the long-term follow-up. Here, CD4 is linked to neoplasm.