Segawa's disease (dopa-responsive dystonia) is caused by a genetic mutation in the gene for GTP cyclohydrolase I, a rate-limiting enzyme for the biosynthesis of tetrahydrobiopterin. Although the causative gene was identified, many issues remain to be addressed in this disease. In this short article, we have described and discussed the relevance of measurement of GTP cyclohydrolase I activity in phytohemagglutinin-stimulated mononuclear blood cells in patients with Segawa's disease. Although the enzyme activity in immune cells may not reflect the activity in the brain, we can evaluate the maximum ability in the induction of GTP cyclohydrolase I in the cell from this method. Under normal conditions without any stimulant, some compensatory mechanism(s) may attenuate the difference in the enzyme activity between patients and normal individuals. Since one allele is inactivated in patients with Segawa's disease, we can distinguish the patients from normal individuals without examining their gene-defect. Because we cannot identify a mutation in the coding region and the exon-intron junction of GTP cyclohydrolase I in about half of patients with Segawa's disease, this biochemical assay has great value in diagnosis of the disease.