To clarify the serum levels of soluble interleukin-2 receptor (sIL-2R) in patients with renal diseases, we examined 281 patients without various renal diseases with or without systemic disease (including 197 patients without systemic diseases and 84 patients with systemic diseases). The sIL-2R level was significantly higher in patients with renal diseases, than in healthy volunteers. In the group of renal diseases, the sIL-2R level in patients with rapidly progressive nephritic syndrome was especially higher than in patients with other renal diseases. In patients without systemic diseases, the serum level of sIL-2R was significantly and positively correlated with the urinary excretion of protein, and the serum levels of creatinine and uric acid, and was negatively correlated with the glomerular filtration rate (GFR). In the patients with systemic diseases, the correlation between the serum levels of sIL-2R and the serum levels of creatinine or GFR were not as strong as observed in the patients without systemic diseases. The serum level of sIL-2R was outside the normal range in patients with systemic diseases with a serum level of creatinine above 2.0 mg/100 ml. These findings suggest that the serum levels of sIL-2R in patients with renal disease may increase and correlate with the impaired renal function, that he increased sIL-2R levels in patients with systemic diseases may be dependent on the activity of systemic disease, and that it may be not useful indicator of diseases activity in patients with systemic diseases when the serum level of creatinine was above 2.0 mg/100 ml.