Leptin plays an essential role in the regulation of body weight by inhibiting food intake and stimulating energy expenditure. In the course of chronic inflammatory diseases such as rheumatoid arthritis, proinflammatory cytokines IL-1 and TNF-α increase expression of the ob gene and leptin secretion. The aim of the study was to evaluate the serum leptin level in rheumatoid arthritis and knee osteoarthritis and assess the correlation between the serum leptin level and BMI, disease activity, degree of tissue damage, bone mass and others markers. 150 women treated in Rheumatology Outpatients Department of the CSK MSWiA were divided into two groups. Group I (n=90) was diagnosed rheumatoid arthritis (RA) according to ARA 1987 criteria. The control group – osteoarthrosis (OA) (n=60) were women with knee osteoarthrosis diagnosed according to Altman’s 1991 criteria. There was not a statistically higher value of serum leptin level in the RA group than in the OA group. There were no statistically significant differences in the serum leptin level depending on stage of disease according to Steinbrocker’s or Larsen’s-Dale scale. The serum leptin level was statistically higher in seronegative patients than in seropositive patients. There were positive correlations between the serum leptin level and BMI, disease duration and level of serum creatinine and a negative correlation with number of thrombocytes in the RA group. There was a positive correlation between the serum leptin level and BMI, patient age, value of haemoglobin, number of erythrocytes and WOMAC index, and a negative correlation with T-score value in the control group.