Malignant phenylketonuria is a rare disease caused by a deficiency in dihydropteridine-reductase which induce a hyperphenylalaninemia and a deficiency of neurotransmitters such as 3,4,dihydroxyphenylalanine (DOPA) and 5 hydroxytryptophan. The case of a patient with malignant phenylketonuria (PKU) who underwent both CT and MR Imaging is reported. CT demonstrated the characteristic calcifications of the basal ganglia. MRI demonstrated areas of hypersignal on T1 images in the basal ganglia, subcortical frontal and occipital white matter and cortex probably corresponding to calcifications. The MR findings are not specific but could be useful in monitoring the diet and neurotransmitter substitution therapy.