The leukotrienes are a family of lipid mediators involved in inflammation and allergy. Leukotriene B4 (LTB4) is a classical chemoattractant which triggers adherence and aggregation of leukocytes to the vascular endothelium at only nM concentrations. In addition, leukotriene B4 modulates immune responses, participates in the host defense against infections, and is a key mediator of PAF-induced lethal shock. The final step in the biosynthesis of leukotriene B4 is catalyzed by leukotriene A4 (LTA4) hydrolase, a unique bifunctional zinc metalloenzyme with anion-dependent aminopeptidase activity. This article describes the most recent developments regarding our understanding of the structure, function, and catalytic mechanisms of leukotriene A4 hydrolase, with emphasis on conclusions drawn from mutagenetic analyses and X-ray crystallography.