Alternating hemiplegia of childhood is a rare disorder of unknown cause associated with progressive neurological deterioration. We report the results of regional cerebral blood flow studies using 99mTc-hexamethylpropylene amine oxime single-photon emission computed tomography in 3 patients. These studies were performed during the hemiplegic attacks (n = 6) and during the symptom-free periods (n = 2). Six single-photon emission computed tomographic studies performed during hemiplegic attacks consistently showed relative hyperperfusion of the contralateral cerebral hemisphere. Two single-photon emission computed tomographic studies performed during the asymptomatic phase showed normal and symmetrical cerebral perfusion. This is the first definite demonstration of unilateral increase of cerebral blood flow in alternating hemiplegia. These findings support the possibility of a relationship between the cause of alternating hemiplegia and migraine.