Abstract： Objective To explore the clinical characteristics and the diagnostic method of maple syrup urine disease (MSUD).Methods From January 2003 to December 2011,a total of 14 000 patients with suspected inherited metabolism diseases were tested.The blood levels of leucine and valine of these patients were detected by tandem mass spectrometry.The urinary level of branched-chain α-ketoacids was tested by gas chromatography-mass spectrometry.And the diagnosis was based on the elevated levels of leucine and valine in blood and branched-chain α-ketoacids in urine.Results Thirty-three MSUD patients were confirmed.Their median age of initial visit was 0.17 years old (range:7 days to 30 years old).The peak onset age of them was 2-30 days old,including 28 cases of neonatal onset (84.8％).The presenting symptoms of 28 cases were feeding difficulties (n =14),poor response,lethargy and seizures.Their median blood levels of leusine and valine (1901 (458-5804) and 600 (315-1617) μmol/L) were significantly higher than their normal levels ((50-300) and (60-250) μmol/L,both P ＜ 0.01).Their urinary levels of 2-OH-isovalericacid,2-keto-isovalericacid,2-keto-3-methylvalericacid,2-keto-isocaproic and acetylglycine (262.5 (5.4-624.3),35.8 (1.9-156.0),133.8 (7.4-611.5),518.7 (17.2-2121.2)and 280.5 (11.0-1087.9) respectively) significantly higher than their normal levels (0,＜ 0.1,0,0,＜0.1 respectively,all P ＜ 0.01).In 5 intermittent MSUD patients,their blood levels of leucine and valine (402 (348-958) and 556 (322-808) μmol/L) were significantly higher than their normal levels (both P ＜ 0.01).The urinary level of 2-OH-isovalericacid was significantly higher than its normal levels (P ＜ 0.01) while the urinary levels of other α-ketoacids were normal.Conclusions The confirmation of MSUD remains difficult because of a lack of specific clinical features.The detections of tandem mass spectrometry and gas chromatography-mass spectrometry may aid its early diagnosis.