Leptin, a 16-kD protein of the ob gene product, is produced by adipose tissue and acts on the hypothalamus to suppress neuropeptide Y secretion which reduces the appetite. It has been demonstrated that serum leptin levels in healthy subjects are correlated with body mass index(BMI). Leptin is also produced by stimulation of inflammatory cytokines such as tumor necrosis factor(TNF)-alpha and interleukin(IL)-1. Rheumatoid arthritis(RA) is one of the chronic inflammatory diseases in which high serum cytokine levels are noted. In this study, we measured serum leptin levels(s-leptin) by RIA kit in 20 healthy subjects (10 males and 10 females) and 49 RA patients(14 males and 35 females) and the markers for joint inflammation such as ESR and CRP. There was no difference in s-leptin between controls(male: mean +/- SD = 5.6 +/- 3.0 ng/ml; female: 7.8 +/- 4.5 ng/ml) and RA patients(male: 4.9 +/- 3.2 ng/ml; female: 9.1 +/- 5.7 ng/ml). S-leptin was correlated with BMI in both healthy subjects and RA patients. However, there was no correlation between s-leptin and the values of ESR or CRP, disease stages in RA patients. In conclusion, s-leptin in RA patients reflects BMI but not joint inflammation.