Leptin may contribute to renal pathology in some situations by stimulating transforming growth factor-beta1 (TGF-beta1) synthesis. The soluble leptin receptor (sOb-R) is a transport protein contributing to binding and activation of circulating leptin. We investigated the interaction between serum and urinary leptin, TGF-beta1, and serum sOb-R levels in 38 patients with minimal change nephrotic syndrome (MCNS) aged between 6 and 12 years and 10 age- and sex-matched healthy controls (group III). Patients were divided into two groups: group I, proteinuria exceeding >40 mg/m(2) per hour and group II, patients in remission. Serum leptin levels in group I were significantly lower than those in group II and group III (P=0.011, P=0.007, respectively). There was a negative correlation between serum leptin levels and proteinuria (r=-0.52, P=0.02) as well as between serum leptin and sOb-R levels (r=-0.82, P=0.000) in group I. Urine leptin and sOb-R levels in group I were significantly higher than in group II (P=0.0021, P=0.001, respectively) and group III (P=0.07, P=0.009, respectively). Serum TGF-beta1 levels in healthy controls (406+/-424 pg/ml) were significantly lower than those in groups I and II (P=0.004, P=0.000, respectively). However, no significant correlation was found between the serum TGF-beta1 and leptin levels in MCNS patients. In conclusion, low serum leptin, high serum TGF-beta1 and sOb-R levels, and elevated urine leptin concentrations were observed at the onset of MCNS. Since long-term proteinuria and leptinuria might be associated with the progression of renal damage, future in vivo and in vitro studies are needed to explain the interaction between these parameters in different types of nephrotic syndrome.