A boy aged 6 years and 3 months developed upper respiratory tract infection and pyrexia 2 months ago and was given oral administration of nimesulide by his parents according to directions. Half an hour later, the boy experienced convulsions and cardiopulmonary arrest, and emergency examination found hypoketotic hypoglycemia, metabolic acidosis, significant increases in serum aminotransferases and creatine kinase, and renal damage. Recovery of consciousness and vital signs was achieved after cardiopulmonary resuscitation, but severe mental and movement regression was observed. The boy had a significant reduction in free carnitine in blood and significant increases in medium-and long-chain fatty acyl carnitine, urinary glutaric acid, 3-hydroxy glutaric acid, isovalerylglycine, and ethylmalonic acid, suggesting the possibility of multiple acyl-CoA dehydrogenase deficiency. After the treatment with vitamin B₂, L-carnitine, and bezafibrate, the boy gradually improved, and reexamination after 3 months showed normal biochemical parameters. The boy had compound heterozygous mutations in the ETFDH gene, i.e., a known mutation, c.341G > A (p.R114H), from his mother and a novel mutation, c.1484C > G (p.P495R), from his father. Finally, he was diagnosed with multiple acyl-CoA dehydrogenase deficiency. Reye syndrome and sudden death symptoms were caused by nimesulide-induced acute metabolic crisis. It is concluded that inherited metabolic diseases may be main causes of Reye syndrome and sudden death, and biochemical and genetic analyses are the key to identifying underlying diseases.