L-3-Hydroxykynurenine (L-3-HK) is a metabolite in the kynurenine pathway, the major route of tryptophan degradation in mammals. Kynurenine 3-monooxygenase, an NADPH-dependent flavin monooxygenase, catalyses the hydroxylation of L-kynurenine to L-3-hydroxykynurenine. 3-hydroxykynurenine can be converted to 3-hydroxyanthranilate by the enzyme 3-hydroxykinureninase. It may also be converted to 4-(2-amino-3-hydroxphenyl)-2,4-dioxobutanoate by the enzyme kynurenine-oxoglutarate transaminase. L-3-Hydroxykynurenine (L-3-HK) is a known generator of highly reactive free radicals. An elevation of L-3-HK levels has been shown to constitute a significant hazard in situations of excitotoxic injury. In particular, L-3-HK may contribute to the neuronal deficits underlying HIV-associated dementia. Pharmacological interventions aimed at decreasing L-3-HK formation may therefore be particularly useful for the treatment of neurological diseases which are associated with an abnormally enhanced flux through the kynurenine pathway.