Notwithstanding the discovery of GCH1 and TH mutations in autosomal-dominant and autosomal-recessive DRD, respectively, a therapeutic trial with levodopa is still the most practical approach to the diagnosis of DRD. The trial needs to be considered in all children with dystonic and/or parkinsonian symptoms or with unexplained gait disorders. Further accumulation of patients with TH-deficient DRD (the mild form of TH deficiency) is necessary to establish the clinical characteristics of this disorder. Regarding GTPCH-deficient DRD, there remain important unresolved issues, including questions of incomplete penetrance of GCH1 mutations, female predominance of affected subjects, and intrafamilial phenotypic variation. A clarification of the mechanism of striatal TH protein loss in GTPCH-deficient DRD may provide a new clue to the pathogenesis of this major form of DRD.