AbstractLocalized proton magnetic resonance spectroscopy with short echo time (TE= 20 ms) was used to investigate biochemical changes in the cerebral white matter of 20 young adult patients (median 19 years) with phenylketonuria (PKU). Results were compared with those of a group of 12 age‐matched healthy volunteers (median 25 years). Concentrations of Nacetyl‐aspartate (NAA) and choline (Cho) relative to creatine (Cr) were unchanged. However, concentrations of inositol (Ins) relative to creatine were found to be significantly lower (P<0.001) in the PKU patients (0.30 ± 0.09 versus 0.57 ± 0.17). Individual inositol concentrations did not correlate with age, diet, serum phenylalanine (Phe) levels or extent of pathological regions in the T,‐weighted images. The lack of correlation with individual data suggests that the decreased inositol concentration could be related to a metabolic deficiency during fetal development. No signal from the phenyl ring protons of phenylalanine was detected in the PKU patients (phenylalanine serum concentration ⩽ 1.27 mM), which suggests that concentration of phenylalanine may be lower in brain than