We report the familial occurrence and apparent autosomal dominant inheritance of alternating hemiplegia of childhood. The proband, a 9-year-old boy, presented with developmental retardation, rare tonic-clonic seizures, and frequent episodes of flaccid alternating hemiplegia that had been presumed to represent postictal paralysis. The hemiplegia spells, which started in his first year, did not respond to multiple antiepileptics. Between attacks, there was choreoathetosis and dystonic posturing. Father, brother, paternal uncle, and paternal grandmother had similar histories of alternating hemiplegia. Investigations included negative CT, metabolic, and coagulation studies. EEG and SPECT 99mTc exametazime scanning failed to reveal any significant slowing or any major changes in cortical perfusion during hemiplegia as compared with nonhemiplegic periods. The karyotype revealed a balanced reciprocal translocation, 46, XY, t(3;9)(p26;q34) in the patient, in all the affected living relatives, and in one apparently unaffected sibling. The asymptomatic mother had a normal karyotype. Analysis of DNA markers was consistent with the karyotype results. Both affected siblings were treated with and responded to flunarizine therapy, with a greater than 70% decrease in attack frequency. Documented flunarizine trough serum concentrations were 28.9 ng/ml in the proband and 6.6 ng/ml in his brother. © 1992 American Academy of Neurology.