The genetics and symptoms of Segawa's disease are described. The latter can show considerable variation, especially if the onset of the condition is delayed. It is usually of autosomal dominant inheritance, but a recessive form can occur. The dominant and recessive forms are caused by a gene mapped to chromosome 14. The fluctuating dystonia is typical, but is not essential for the diagnosis. Affected children can suffer from sleep disorders, such as excessive sleepiness and nightmares. In some children with this condition mutations in the GCH-1 gene coding for guanosine triphosphate cyclohydrolase 1 have been found. The enzyme catalyses the first step in the biosynthesis of tetrahydrobiopterin. A point mutation in the tyrosine hydrolase gene has been found in some of the recessive forms, and the gene mapped to chromosome 11, but this cannot be called Segawa's disease. The deficiency of guanosine triphosphate cyclohydrolase 1 causes a defect in serotonin metabolism and in the biosynthesis of tetrabiopterin and a disturbance of dopamine metabolism. This leads to a deficiency of dopamine in the striatum, and to the motor dysfunction of the syndrome. The diagnosis can be established by cerebrospinal fluid examination, and confirmed in some patients by genetic studies. Treatment is with levodopa, and the results are dramatic.