A consistent neurochemical abnormality in Parkinson's disease (PD) is degeneration of dopaminergic neurons in substantia nigra, leading to a reduction of striatal dopamine (DA) levels. As tyrosine hydroxylase (TH) catalyses the formation of L-DOPA, the rate-limiting step in the biosynthesis of DA, the disease can be considered as a TH-deficiency syndrome of the striatum. Similarly, some patients with hereditary L-DOPA-responsive dystonia, a neurological disorder with clinical similarities to PD, have mutations in the TH gene and decreased TH activity and/or stability. Thus, a logical and efficient treatment strategy for PD is based on correcting or bypassing the enzyme deficiency by treatment with L-DOPA, DA agonists, inhibitors of DA metabolism, or brain grafts with cells expressing TH. A direct pathogenetic role of TH has also been suggested, as the enzyme is a source of reactive oxygen species (ROS) in vitro and a target for radical-mediated oxidative injury. Recently, it has been demonstrated that L-DOPA is effectively oxidized by mammalian TH in vitro, possibly contributing to the cytotoxic effects of DOPA. This enzyme may therefore be involved in the pathogenesis of PD at several different levels, in addition to being a promising candidate for developing new treatments of this disease.