Mutations in the GTP‐cyclohydrolase I(GCH) gene have been identified as a cause of two disorders: autosomal dominant hereditary progressive dystonia/dopa‐responsive dystonia (HPD/DRD) and autosomal recessive GCH‐deficient hyperphenylalaninemia (HPA). Detailed clinical descriptions and genetic analysis of patients with phenotypes intermediate between HPD/DRD (mild) and GCH‐deficient HPA (severe) have not been reported. We conducted genomic DNA sequencing of the GCH gene in two patients (cases 1 and 2) manifesting generalized dystonia responsive to levodopa and severe developmental motor delay. In the pedigree of Patient 1, there were HPD/DRD patients in three generations preceding the index case. Patients 1 and 2 were compound heterozygotes with maternally and paternally transmitted mutations in the coding region of the GCH gene. In both compound heterozygotes, tetrahydrobiopterin (BH₄) levels in cerebrospinal fluid were lower than those in HPD/DRD. Administration of BH₄, in addition to levodopa, further improved the symptomatology of Patient 1. Our data demonstrate a new phenotype of GCH deficiency associated with compound heterozygosity for GCH gene mutations and suggest the usefulness of combined BH₄ and levodopa therapy for this disorder.