 The comparison of changes in peripheral T-cell subpopulations at different stages of rheumatoid arthritis (RA) development may be important to understand the pathomechanism and to elucidate the course of RA. So far, there have been no comprehensive studies regarding the proportions of T cells in early and long‑lasting RA.
 The aim of this study was to assess the proportion of the main peripheral T-cell subpopulations in patients at various stages of RA development.
 We enrolled 75 patients who were divided into 4 subgroups depending on the diagnosis: undifferentiated arthritis (UA), which later developed into RA (UA‑RA) and other diseases (UA‑non‑RA); clinically confirmed untreated RA; long-term treated RA; and the control group. Flow cytometry was used to assess T-cell subpopulations.
 Patients with clinically confirmed untreated RA differed (P <0.05) in the proportion of CD4+ T-cell subpopulations expressing activation markers compared with controls (CD69, CD25, HLA‑DR, CD95) and UA patients (CD95). Untreated RA patients had the highest proportion of regulatory CD4+ T cells compared with control and other groups. The percentage of CD28- T cells was higher only in the group with clinically confirmed RA but not in those with early RA (at the UA stage).
 The peripheral T lymphocyte phenotype in very early RA is not similar to that observed in clinically‑confirmed RA. Patients with a confirmed diagnosis of RA can be easily differentiated based on the absolute numbers of the main T-cell subpopulations; however, the percentage of the main T-cell subpopulations do not discriminate those patients in the UA cohort who will develop RA.