This study primarily aims at comparing the sensitivity and specificity of overnight 5HIAA value, 24-hour 5HIAA value and plasma 5HIAA value, in patients with small-intestine neuroendocrine tumors. Secondary objectives include the correlation of 5HIAA values with chromogranin A, carcinoid syndrome and tumor burden and the evaluation of the compliance to the diet and its correlation with 5HIAA values.