Hereditary progressive dystonia with marked diurnal fluctuation (HPD) (also known as dopa responsive dystonia) is a dystonia with onset in childhood that shows a marked response without any side effects to levodopa. Recently the gene for dopa responsive dystonia (DRD) was mapped to chromosome 14q. Here we report that GTP cyclohydrolase I is mapped to 14q22.1–q22.2. The identification of four independent mutations of the gene for GTP cyclohydrolase I in patients with HPD, as well as a marked decrease in the enzymes activity in mononuclear blood cells, confirms that the GTP cyclohydrolase I gene is a causative gene for HPD/DRD. This is the first report of a causative gene for the inherited dystonias.