We studied the clinical usefulness of I123-IMP SPECT in 50 pediatric patients with CNS disorders, which were categorized into the convulsive disorder group (n = 20), the cerebrovascular disorder group (n = 10), the acute encephalopathy or CNS infection group (n = 10), the metabolic or degenerative disorder group (n = 6), the congenital abnormality group (n = 2) and the migraine group (n = 2). The findings obtained were compared with those of cranial CT. I123-IMP SPECT revealed abnormal findings in 45 out of the 50 patients (90%), although cranial CT showed abnormal findings in only 24 patients (48%). This difference was statistically significant (p less than 0.01). In all groups except the migraine, we could find abnormal findings in more than 90% of the patients. Out of 28 patients without focal findings on the initial CT scanning, I123-IMP SPECT showed focal abnormalities in 26 patients (93%). Moreover in many patients with focal neurological abnormalities, we found focal abnormalities of I123-IMP SPECT related with neurological abnormalities of the patients. From these findings, we think I123-IMP SPECT might be better to CT scanning in examining a localized lesion. It was found that in many patients with focal abnormalities in CT scanning, I123-IMP SPECT showed larger abnormalities in CT scanning. By using I123-IMP SPECT we might be able to study the blood perfusional state surrounding the abnormal area shown by CT. In 3 patients with acute cerebrovascular disorders, I123-IMP SPECT revealed abnormal findings 3 to 11 days earlier than cranial CT.I123-IMP SPECT might be useful for early recognition of the pathological state.(ABSTRACT TRUNCATED AT 250 WORDS)