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.