Serotonin (5-hydroxytryptamine, 5-HT) is readily secreted in patients with carcinoid tumors, especially arising from the midgut. Although serotonin assay in human plasma or whole blood has been extensively studied, serotonin assay in human serum by liquid chromatography-tandem mass spectrometry (LC-MS/MS) has received much less attention. In this study, a simple and reliable LC-MS/MS method based on one step protein precipitation for sample pretreatment was developed for clinical assay of serum serotonin. Namely, 150 mu L of serum was mixed with 50 mu L of internal standard (IS) and 200 mu L of 4 wt.% 5-sulfosalicylic acid (SSA) solution for protein precipitation. The supernatant after centrifugation was analyzed directly without further treatment. This method was validated for consistent linearity from 0.94 to 240 ng/mL with CVs <= 11.7%, good recovery in the range of 87.5%-104%, excellent analyte stability and low carryover. No obvious matrix effect was observed. Intra- and inter-day imprecision were below 8.03% and 11.5% respectively. Dilution linearity was verified with satisfying linearly dependent coefficients (r(2) = 0.9937). The reference interval of serotonin was established from 126 results derived from subjects without carcinoid tumors. Therefore, apart from development of a serum serotonin assay by the LC-MS/MS method, the reference interval (RI) of 5-HT has also been established for clinical testing in patients with carcinoid tumors. In addition, this method has been successfully used in our laboratory, indicating that this robust LC-MS/MS assay with simple sample preparation and short analysis time could offer inspiring potential for clinical testing of 5-HT in routine clinical laboratories.