Autosomal dominant DOPA-responsive dystonia (DRD) is usually caused by mutation in the gene encoding guanosine triphosphate-cyclohydrolase I (GTPCH I). We studied 22 families with a phenotype of levodopa-responsive dystonia by sequencing the six coding exons, the 5'-untranslated region and the exon-intron boundaries of the GTPCH I gene. Eleven heterozygous mutations were identified, including five missense mutations, one splice site mutation, two small deletions and two nonsense mutations, in 12 families that included 27 patients and 13 asymptomatic carriers. Six mutations were new and five had already been reported. Four of the mutations caused truncation of the GTPCH I protein. One family carried a base-pair change in the 5'-untranslated region, not detected in controls, that could be responsible for the phenotype. Three of the remaining 10 families had deletions in the parkin gene on chromosome 6, underlining how difficult it is to distinguish, in some cases, between DRD and parkin mutations. No mutations were identified in seven families. The clinical spectrum extended from the classical DRD phenotype to parkinsonism with levodopa-induced dyskinesias, and included spastic paraplegia as well as the absence of dystonia.