Azotemic patients are usually characterized by a state of so-called preactivation resulting in excessive expression of interleukin 2 receptors (IL-2R) on T lymphocytes. The etiological mechanism of this preactivation is, however, still speculative. We studied the serum level of the soluble form of IL-2R (sIL-2R) in azotemic patients on either hemodialysis (HD) (n = 49) or continuous ambulatory peritoneal dialysis (CAPD) (n = 45). Both patient groups had significantly higher sIL-2R levels (1,750+/-664 U/ml in the HD group and 1,769+/-647 U/ml in the CAPD group, respectively) p < 0.00001 as compared to the normal control group (511+/-436 U/ml). However, there was no significant difference between the levels of the HD and CAPD group patients. When clinical parameters were studied for their influence on sIL-2R levels, none of the following caused any significant changes: blood transfusion, type of dialyzer used, type of dialysis fluid used, treatment with erythropoietin, hepatitis B infection, or liver function profile. We conclude that chronic renal failure per se is the major cause of the preactivation of T lymphocytes. The modes of treatment and various clinical variables in these patients have no significant influence on sIL-2R levels.