Dopa-responsive dystonia (DRD) was originally described in a series of Japanese patients, but is now increasingly recognized in other countries. Recently the GTP cyclohydrolase I (GTPCH) gene was isolated as the first causative gene for dopa-responsive dystonia (DRD). Mutations were identified in three Japanese families with autosomal dominantly inherited DRD and in one sporadic Japanese patient. Characterisation of the exon-intron boundaries of this gene has now allowed the analysis of mutations at the level of genomic DNA. Amplifying all six exons, we analyzed the GTPCH gene in nine British families with 33 affected family members and in three sporadic cases and found six new mutations. Only point mutations were found, causing a stop codon in one family and an amino acid change in highly conserved regions of the gene in a further four families and in one sporadic case. None of these mutations were detected more than once and none of the mutations previously described were found in our patients. No mutations were identified in four families and in two sporadic cases.