GTP cyclohydrolase I activity in mononuclear blood cells from patients with juvenile parkinsonism (JP) was found to be normal compared to healthy controls. The normal activity in JP contrasts strongly with the decreased activity of 2-20% normal levels in hereditary progressive dystonia with marked diurnal fluctuation (HPD) or dopa responsive dystonia (DRD). The result indicates that the decreased dopamine level in the basal ganglia in JP is not due to decreased activity of GTP cyclohydrolase I, the enzyme for the biosynthesis of the tetrahydrobiopterin cofactor of tyrosine hydroxylase (TH), and the enzyme activity in mononuclear blood cells could be a reliable method for differential diagnosis between JP and HPD/DRD.