The ob gene product leptin is thought to be a key regulator of food intake and body weight. Patients having advanced chronic renal failure (CRF) have markedly higher serum leptin levels. It is not known whether the increase in leptin levels in CRF is caused by a decreased plasma clearance and/or increased production. In the present study serum leptin levels and glomerular filtration rate (GFR) were measured in 219 patients having various degrees of renal failure. In addition, serum leptin levels, C-reactive protein (CRP), body composition (by dual-energy x-ray absorptiometry) and ob gene expression (by in situ hybridization histochemistry) were determined in 15 patients with advanced CRF. Seven of the patients were re-evaluated following 12 months of peritoneal dialysis (PD) treatment. Serum leptin levels correlated negatively to GFR (r = -0.26; P < 0.0001). The ob gene expression was significantly lower in patients with CRF than in healthy controls. A negative correlation between serum leptin levels and ob gene expression (r = -0.66; P < 0.05) was found in patients with CRP < 25 mg/liter. The ob gene expression (20.0 +/- 1.8 vs. 15.0 +/- 1.0 nCi/g; P < 0.05) was significantly higher in 5 patients with CRP > 25 mg/liter than in 10 patients with CRP < 25 mg/liter. Following 12 months of PD, the amount of body fat increased by 30% while the ob gene expression remained unchanged. The present study shows a correlation between serum leptin levels and GFR, and our results suggest that elevated leptin levels, due to a decreased plasma clearance, down-regulate the expression of the ob gene. We also found that an ongoing inflammation stimulates ob gene expression in patients with CRF. Therefore, it is suggested that the hyperleptinemia induced feedback inhibition of ob gene expression is overcome by inflammatory cytokines.