We studied the changes in cellular immunity in patients in replacement dialysis therapy (RDT) and examined the relationship between T-lymphocyte function and plasma levels of parathyroid hormone (PTH). In a preliminary study we found that increased plasma levels of PTH were associated with a decrease of T-lymphocytes and CD4, an increase in CD8 and a reduction in the ratio of CD4 to CD8. In the present study we examined the relationship between plasma levels of PTH, interleukin 2 receptors (IL-2R) and soluble human CD8 (S-CD8). We studied 54 patients divided into two groups: 26 patients with normal levels of PTH and 28 patients with increased levels of PTH. We found a significant reduction in total T-lymphocytes in both groups as compared to controls, with an inverse correlation between total T-lymphocytes and plasma PTH in the second group (R = -0.52). There was an increase in IL-2R in the group II as compared to the controls and also in the total population of uremic patients with a linear correlation between levels of IL-2R and PTH (R = 0.6). The levels of S-CD8 showed a significant increase in both groups with a linear correlation between levels of SC-D8 and PTH (R = 0.63). No specific differences were seen between patients treated with and without 1,25-dihydroxyvitamin D3. The elevated levels of PTH affect the lymphocyte function and are associated with change in cellular immunity with reduction in total number of T cells, and increases in levels of CD8, S-CD8 and IL-2R.