To explore the molecular etiology of two disorders caused by a defect in GTP cyclohydrolase I--hereditary progressive dystonia with marked diurnal fluctuation (HPD), also known as dopa-responsive dystonia (DRD), and autosomal recessive GTP cyclohydrolase I deficiency--we purified and analyzed recombinant human wild-type and mutant GTP cyclohydrolase I proteins expressed in Escherichia coli. Mutant proteins showed very low enzyme activities, and some mutants were eluted at a delayed volume on gel filtration compared with the recombinant wild-type. Next, we examined the GTP cyclohydrolase I protein amount by western blot analysis in phytohemagglutinin-stimulated mononuclear blood cells from HPD/DRD patients. We found a great reduction in the amount of the enzyme protein not only in one patient who had a frameshift mutation, but also in an HPD/DRD patient who had a missense mutation. These results suggest that a dominant-negative effect of chimeric protein composed of wild-type and mutant subunits is unlikely as a cause of the reduced enzyme activity in HPD/DRD patients. We suggest that reduction of the amount of the enzyme protein, which is independent of the mutation type, could be a reason for the dominant inheritance in HPD/DRD.