Background and Objectives The purpose of this study was to investigate whether a brief cardiopulmonary by-pass, with mild hypothermia and normal flow, would have any deleterious effects on brain metabolism in children. This is the first study using localized in vivo proton magnetic resonance spectroscopy 1H-MRS in brains of children with ASD undergoing open-heart surgery OHS . Subjects and Methods Seven children undergoing surgical closure of ASD, with mild hypothermia and normal flow cardiopulmonary by-pass, were studied. Their ages ranged from 18 to 47 months, and body weights ranged from 11 to 17 kg. We performed 1H-MRS before OHS, and 1 3 months after, then compared the values of brain metabolite ratios before and after surgery. The values from our patients were compared to those from 12, age-matched, normal children. All MR imaging, and localized 1H-MRS, were performed using a GE 1.5T SIGNA system General Electric Medical System, Milwaukee, USA . Image guided STEAM-spectra were obtained from, the parietal white matter PWM , and occipital gray matter OGM , with TE of 30 msec, and TR of 3 sec, using a PROton Brain Exam PROBE General Electric Medical System, Milwaukee, USA . Results All metabolite ratios, measured by 1H-MRS, from patients before surgery showed no significantly difference from postoperative values. However, choline/creatine ratios, obtained from PWM, were higher in both pre-and postoperative ASD patients, as compared to those of normal children. Conclusion We concluded, that brain metabolism, measured by 1H-MRS, does not change significantly after OHS, for children with ASD, with mild hypothermia and normal flow. Korean Circulation J 2002 ; 32 2 : 155-162