Background: Hyperpyrexia in childhood occasionally triggers some serious febrile illnesses such as febrile convulsions, deliria, or acute encephalopathies including influenza-associated encephalopathy (IAE).It was reported that hyperpyrexia might be responsible for impaired fatty acid oxidation (FAO) in IAE.We investigated the effect of heat stress that might be related to encephalopathy on FAO.Methods: Cell lines from 15 cases of FAO disorders including CPT2 deficiency, VLCAD deficiency, MTP deficiency or MCAD deficiency as well as 6 controls were examined for acylcarnitine (AC) profiles by the in vitro probe assay using MS/MS.The AC profiles in cells cultured at 37℃ and 41℃ were compared in different conditious.Results and Discussions: 1) Heat stress (at 41℃ compared with at 37℃) increased acetylcarnitine (C2), which reflects the production of acetyl-CoA in all cells from FAO disorders and controls.It is suggested that heat stress generally facilitates FAO.2) In controls and MCAD deficiency, ACs from the short to long-chain species decreased.3) In long-chain FAO disorders including CPT2-VLCAD-MTP-deficiencies, long-chain ACs (at least C16) significantly increased.4) In glutaric acidemia type 2 (GA2), which is caused by a defect of ETF or ETF-DH and results in multiple acyl-CoA dehydrogenation deficiency, the short-to medium-chain ACs (C4 to C 10) decreased at 41℃, while the long-chain ACs (C12 to C 16) significantly increased.These findings indicate that heat stress might inhibit long-chain FAO.Patients with impaired FAO may be more vulnerable to heat stress.It may be concluded that some acute encephalopathy in childhood is related to temporarily or congenitally impaired FAO under heat stress.