To determine the relation between sudden unexpected death in infancy （SUDI） or apparent lifethreatening events （ALTE） and inborn errors of metabolism （IMD）, we investigated clinical and biochemical features in patients presenting with SUDI or ALTE, who were diagnosed with having hidden IMD. Subjects were infants between aged from 2 days and 3 years, detected during the period between January 2004 and December 2014. The diagnosis of IMD was based on the findings of gas chromatography/mass spectrometry and/or tandem mass spectrometry. IMDs were detected in 3 of 239 （1.2%） patients with SUDI, and in 22 of 219 （10.0%） patients with ALTE. These patients had a history of some prodromal illness and/or abnormalities of routine laboratory tests. When a case with SUDI or ALTE is encountered at an emergency unit, it is essential to take detailed medical histories and to measure biochemistry tests including mass spectrometric analysis as well as genetic testing.