5-Lipoxygenase (5LO), by producing leukotrienes, is a proinflammatory enzyme, and there is evidence suggesting that it is up-regulated with aging and may be involved in Alzheimer’s disease (AD). In this paper, we studied the effect of 5LO-targeted gene disruption on the amyloid phenotype of a transgenic mouse model of AD, the Tg2576. Amyloid- (A) deposition in the brains of Tg2576 mice lacking 5LO was reduced by 64–80% compared with Tg2576 controls. This reduction was associated with a similar significant decrease in A levels measured by sandwich ELISA. Absence of 5LO did not induce any significant change in amyloid- precursor protein (APP) levels and processing, or A catabolic pathways. Furthermore, in vitro studies showed that 5LO activation or 5LO metabolites increase, whereas 5LO inhibition decreases, A formation, secondary to correspondent changes in -secretase activity. These data establish for the first time a novel functional role for 5LO in the pathogenesis of AD-like amyloidosis, thereby modulating -secretase activity. Our work suggests that pharmacological inhibition of 5LO could provide a novel therapeutic tool for AD.—Firuzi, O., Zhuo, J., Chinnici, C. M., Wisniewski, T., Praticò, D. 5-Lipoxygenase gene disruption reduces amyloid- pathology in a mouse model of Alzheimer’s disease.