PURPOSE: The purpose of this study was to evaluate MR findings among patients with phenylketonuria (PKU). MATERIALS AND METHODS: Eleven patients with biochemically documented PKU underwent MR imaging ; In nine, the typical classic form was seen, and two were atypical. We evaluated signal intensity, the distribution of abnormal signal intensity, the extent of brain atrophy, and possible clinical correlation between IQ scores of the patients and abnormal signal intensity. RESULTS: Varying degrees of symmetrical high signal intensity were noted on T2-weighted sequences in the parietal white matter of six patients ; the periventricular deep white matter was most seriously affected, but there was no evidence of brain atrophy. In one advanced case, high signal intensity of both the parietal and frontal lobes was seen on T2-weighted images, and brain atrophy and gyriform enhancementon contrast enhanced T1-weighted images. In five patients, findings were normal. no abnormality was found in the basal ganglia, brain stem or cerebellum. There was no correlation between IQ level and severity of high signal intensity. CONCLUSION: Although MR findings were nonspecific, PKU patients showed symmetrical high signal intensity, predominantly in the peritrigonal region. In the advanced case, the lesion, as seen on T2-weighted images, extended to the periventricular and subcortical white matter.