Soluble interleukin 2 receptor (sIL2R) may be used as an index of immune perturbation. We report on the correlation between the serum levels of sIL2R, as assessed with a sandwich-ELISA, and disease activity in 61 patients with Systemic Lupus Erythematosus (SLE) and in 15 with systemic idiopathic vasculitis (SIV). The mean levels of sIL2R in SLE and SIV patients were significantly higher than in healthy controls and higher values were detected in patients with active disease or severe organ involvement or infection. We also studied patients with clinically silent SLE, characterized by the presence of several immunologic abnormalities. The sIL2R mean level in this group did not differ from that of quiescent SLE patients, suggesting that immunologic alterations are present even in inactive SLE. Finally, the sIL2R assay showed higher sensitivity and specificity than most of the common immunologic parameters.