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Background: Neuroendocrine tumors (NETs) are a group of neoplasms, in which circulating biomarkers chromogranin A (CgA), serotonin, and N-terminal brain natriuretic pro-peptide (NT-proBNP), used as a marker of carcinoid heart disease, are especially important in the diagnosis and monitoring. A number of pre-analytical factors, including patients' body mass index (BMI), have an impact on NETs biomarker concentrations. Aim: To perform a comparative analysis of serum concentrations of CgA, serotonin and NT-proBNP in patients with NETs of various locations with normal body weight and with overweight or obesity. Methods: This cross-sectional study included 94 patients with NETs of various locations and 78 provisionally healthy individuals without cancer and cardiovascular disorders, matched by gender and age to the patients. Serum biochemical markers were measured before a course of chemotherapy/biotherapy or surgery with the use of standardized enzyme-linked immunosorbent assays Chromogranin A NEOELISA (Eurodiagnostica), Serotonin ELISA (IBL), and the electrochemiluminescent assay (Cobas e601 analyzer, Roche). All NETs patients were divided into 2 subgroups: 46 patients with normal bodyweight (BMI 18 to 24.9) and 48 patients with overweight/obesity (BMI ≥ 25). Results: The median BMI in the NET patients did not vary depending on their tumor extension, functional activity and malignancy grade. The median concentrations of CgA (150 ng/ml), serotonin (188 ng/ml) and NT-proBNP (117 pg/ml) in the NET patients with obesity and overweight were significantly lower (p = 0.008, p = 0.005 and p = 0.012, respectively) than in the NET patients with normal body weight (769 ng/ml, 704 ng/ml and 197 pg/ml, respectively). In the control group, significantly (p = 0.0001) lower levels in the subgroup with BMI ≥ 25 were obtained only for serotonin. The ROC analysis showed a decrease of the diagnostic efficiency of NET biomarkers in obesity: in the normal BMI group AUC (CgA) was 0.87 and AUC (serotonin) 0.78, whereas in those with BMI ≥ 25, the AUC (CgA) and AUC (serotonin) were 0.81 and 0.62, respectively. Conclusion: Obesity may make it difficult to identify biochemical markers of NETs – CgA, serotonin and NT-proBNP.