Cerebral magnetic resonance imaging (MRI) has revealed white matter changes in patients with phenylketonuria (PKU), an inborn error of metabolism with increased plasma phenylalanine level. Because the significance of these lesions is unknown, this study was undertaken to determine whether glucose metabolism was depressed in cerebral white matter MRI changes in patients with PKU. Four patients with PKU and nine healthy volunteers with an average age of 23 y (range 19-26 y) and 23 y (range 20-27 y), respectively, were studied. The IQ of patients with PKU was between 58 and 97. Cerebral MRI and positron emission tomography images with 18F-deoxyglucose were obtained, and arteriovenous differences for oxygen and glucose as well as cerebral blood flow was measured simultaneously to determine global cerebral oxygen and glucose metabolism. Cerebral MRI revealed that all patients with PKU had white matter changes with characteristic localization. In patients with PKU, regional glucose metabolism was 36% lower in the anterior periventricular areas, 0.14 ± 0.06 compared with 0.22 ± 0.04 μmol · g-1 · min-1 in controls (mean ± SD, p < 0.05, Mann-Whitney). Further, the ratio between glucose metabolism in the affected white matter and the cortex was 14% lower in the patients, decreasing from 0.57 ± 0.05 to 0.48 ± 0.06 (p < 0.05). Global cerebral blood flow, oxygen and glucose consumption were similar in the two groups. In conclusion, regional glucose metabolism is lower in MRI- demonstrated white matter changes. In mildly intellectually impaired patients with PKU, global cerebral glucose and oxygen metabolism remain intact.