The purpose of this study is to examine serum concentration of leptin and that of the soluble form, the Ob-Re receptor, in patients with colorectal cancer, as well as to examine the level of leptin mRNA and that of its receptors, Ob-Ra and Ob-Rb, in large intestine specimens collected from patients with colorectal cancer, depending on cancer clinical and pathological progression and BMI. A total of 146 patients with colorectal cancer in a I-IV stage scale according to the TNM Classification were enrolled. The patients were divided into two groups according to BMI calculations based on body weight and height: a Study group (BMI greater than or equal to 25 kg/m2) of 75 patients aged 57 plus or minus 4.5 years and a Control group (20 less than BMI less than 25 kg/m2) of 71 patients aged 60 plus or minus 5 years. The experimental part of the work was performed in two stages: Stage I regarding the assay of leptin concentration and that of its soluble receptor, Ob-Re, in the serum of patients with the use of the ELISA method; and Stage II to determine the number of leptin mRNA copies and two isoforms of leptin receptors, Ob-Ra and Ob-Rb, using the QRT-PCR method in tissue specimens collected from 146 patients. In our results the concentration of serum leptin and Ob-Re was not dependent on the stage of clinical and pathological progression of the cancer. There was a statistically significant higher serum leptin level in colon cancer patients who were overweight or obese compared to patients with normal weight. No presence of mRNA of the gene encoding leptin was found in tissues collected from colorectal cancer patients. The number of mRNA copies of Ob-Rb was statistically significantly higher in all the study groups compared to the reference tissues.