Purpose: This study was designed to investigate the changes in soluble interleukin-2 receptor (sIL-2R) level in sera and urines of children with primary nephrotic syndrome, eliminating the confounding effects of age, proteinuria, and steroid treatment. Methods: Soluble IL-2R was measured by ELISA in sera and urines from patients with minimal change nephrotic syndrome or focal segmental glomerulosclerosis as well as from healthy controls. The serum levels and urinary sIL-2R/creatinine ratios were compared between control group and the 12 patient groups divided by their ages (0-1, 2-4, over 5 years), and presence or absence of proteinuria and/or steroid treatment (PU+Tx-, PU+Tx+, PU-Tx+, PU-Tx-). Results: Though the differences were not statistically significant probably because of the small numbers, serum sIL-2R levels seemed to be higher in younger age groups both in patients and control group. Nephrotic children did not show higher serum levels than normal children. Among the patients, proteinuric condition seemed to raise and steroid treatment tended to suppress the serum sIL-2R levels. Urinary sIL-2R/creatinine ratios were higher in younger age groups, more significantly in patients (P<0.001). Proteinuria and steroid treatment affected the urinary sIL-2R/creatinine ratios by the same way as the serum sIL-2R levels. Serum sIL-2R levels and urinary sIL-2R/creatinine ratios were not different between groups of different histologic findings or steroid responsiveness (P>0.05). Conclusion: Serum sIL-2R levels and the urinary sIL-2R/creatinine ratios were higher in younger age, and they were not higher in nephrotic patients compared to control group. The patients in relapse showed higher levels, while the levels were suppressed with steroid treatment. In proteinuric state, urinary sIL-2R/creatinine ratios reflected serum sIL-2R levels.