Genes for paroxysmal choreoathetosis have been localized to chromosomes 1p and 2q. We have reinvestigated one of the classic large autosomal-dominant pedigrees of the dystonic Mount-Reback type of paroxysmal choreoathetosis 20 years after its first assessment. These patients prefer diazepam for both prevention and treatment of attacks and did not develop addiction on an intermittent regime. Migraine occurred in a third of the patients. Genetic data localized the underlying mutation to the FPD1 locus (familial paroxysmal dyskinesia type 1) on chromosome 2q and support locus homogeneity for the Mount-Reback syndrome. The data also refine the FPD1 candidate region to 3.6 cM between the markers D2S164 and D2S2359, which may facilitate the investigation of the role of the candidate ion channel gene SLC2C.