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 less than 0.001) and in osteoarthritis (298 +/- 33 U/ml; P less than 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 U/ml; P less than 0.001) and systemic lupus erythematosus (active vs inactive, 2108 +/- 489 vs 499 +/- 75 U/ml; P less than 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 conditions.