Congenital lactic acidosis form a large group of disorders that are commonly associated with profound neurological dysfunction. Difficulties are frequently encountered in establishing a diagnosis, and the mechanisms underlying brain damage are poorly understood. We have performed proton magnetic resonance spectroscopy (1H-MRS) on 24 patients under investigation for suspected metabolic disorder, and have compared the MRS observations of brain lactate with measurements of cerebrospinal fluid (CSF) lactate. We have shown good concordance between the two types of observation, confirming the value of the CSF measurements. Regional variations in brain lactate are detected in some cases, and these may help to elucidate the mechanisms underlying selective brain damage.