We examined the autopsied brains of two parkinsonian patients who had malignant syndrome (MS). Neopterin and biopterin contents, and GTP cyclohydrolase I activity were measured in various region of the brain. We found relatively higher GTP cyclohydrolase I activities in the hypothalamus compared with other regions of the brain from patients with MS. This finding suggested a possible involvement of biopterin metabolism in pathophysiology of MS. This is the first report on biopterin metabolism in the brains of patients with MS.