BACKGROUND: The assessment of hormonal function of neuroendocrine neoplasm (NEN) is an important stage in the diagnosis and monitoring of these diseases treatment. Objective of this study was to analyze the results of urinary excretion of 5-hydroxyindoloacetic acid (5-HIAA) in patients with carcinoid syndrome treated with somatostatin analogues, depending on the histologic maturity, degree of liver involvement and stage of the disease. METHODS: The final group comprised of 41 patients. All patients were subject to surgical removal of the primary site. Presence of hepatic metastases was determined in all patients. All patients were treated with somatostatin analogues. The 5-HIAA urine excretion was determined using the ELISA immunoenzymatic method. RESULTS: The mean excretion of 5-HIAA in patients with histological maturity grade G1 was 45.64 mg/24h, while in the group G2 the mean excretion was 108.41 mg/24h and was higher than in the group G1 (p=0.003). In the analysis of 5-HIAA value depending on the degree of liver involvement, the mean value of 5-HIAA excretion in patients with 10% liver involvement was 38.99 mg/24h, whereas in patients with 25% liver involvement this value was considerably higher and amounted 131.00 mg/24h (p< 0.001). In patients with disease progression the mean excretion was 117.37 mg/24h compared to the group of patients with stabilization of the disease, where the mean value was lower and amounted to 39.39 mg/24h (p<0.001). CONCLUSION: Assessment of 5-HIAA excretion in patients with carcinoid syndrome is of considerable significance in the diagnostics and monitoring of the treatment.