In this study we investigated the serum levels of a released soluble form of the interleukin-2 receptor (sIL-2R) in 42 patients with rheumatoid arthritis and in 12 cases of systemic lupus erythematosus. Data were evaluated in relationship to the clinical phase and compared with those observed in normal controls (N=56) and in osteoarthritis (N = 7). Increased levels were observed in both rheumatoid arthritis (mean ± SE, 604±49 U/ml) and systemic lupus erythematosus (1438±481 U/ml). These values were significantly higher than in control (256±15 U/ml;P<0.001) and="" in="" osteoarthritis="" (298±33="">P<0.001) groups.="" in="" addition,="" the="" highest="" values="" were="" associated="" with="" the="" active="" phases="" of="" both="" rheumatoid="" arthritis="" (active="" vs="" inactive,="" 771±78="" vs="" 451±39="">P<0.001) and="" systemic="" lupus="" erythematosus="" (active="" vs="" inactive,="" 2108±489="" vs="" 499±75="">P<0.001). our="" findings="" suggest="" that="" the="" detection="" of="" sil-2r="" in="" rheumatoid="" arthritis="" and="" in="" systemic="" lupus="" erythematosus="" may="" represent="" a="" good="" marker="" of="" disease="" activity,="" which="" indirectly="" indicates="" the="" ongoing="" activation="" and/or="" proliferation="" of="" immunoreactive="" cells="" which="" are="" involved="" in="" the="" pathogenetic="" events="" of="" these="" autoimmune="">