In 10 patients after kidney transplantation the T cell subpopulations (CD4, CD8) and the number of activated lymphocytes (IL-2R and transferrin receptor+, 4F2+, HLA-DR+, CD3+) in the peripheral blood were determined. In 9 out of 9 cases with rejection a clear immune activation could observed. In patients with CMV-associated rejection crisis the CD4/CD8 ratio was significantly higher compared with patients with rejection crisis but without CMV infection. In all patients with CMV infection (n = 5) an increase of activated lymphocytes was observed. In a clinical-immunological monitoring the dynamics of the CD4/CD8 ratio, the expression of activation marker (especially CD3+, HLA-DR+ and 4F2+) and the absolute number of lymphocyte subpopulations should included.