Objectives: Free radicals play an important role in the onset and progression of many diseases. The aim ofthis study was to investigate the contribution of oxidative stress in the pathology of aggressive (AgP) andchronic (CP) periodontitis and its relation with the clinical periodontal status.Methods: Eighty subjects were divided into two groups: 20 patients with AgP and 20 patients with CP withtheir 20 corresponding matched controls, based on clinical attachment loss (CAL), probing pocket depth(PPD), and bleeding on probing (BOP). Saliva reactive oxygen species (ROS), lipid peroxidation, and nonenzymaticantioxidant defences were measured by luminol-dependent chemiluminescence assay, asthiobarbituric acid-reactive substances (TBARs) and total radical-trapping antioxidant potential (TRAP),respectively. Pearson?s correlation and multivariate analysis were used to determine the relationshipbetween ROS and TBARs and the clinical parameters.Results: ROS and TBARs were increased in AgP while TRAP was decreased, comparing with CP. In AgP, astrong and positive correlation was observed between ROS and TBARs and they were closely associatedwith CAL and PPD.Discussion: In AgP, but not in CP, oxidative stress is a high contributor to periodontal pathology and it isclosely associated with the clinical periodontal status.Fil: Acquier, Andrea Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas; ArgentinaFil: de Couto Pita, Alejandra Karina. Universidad de Buenos Aires. Facultad de Odontología. Cátedra de Farmacología; ArgentinaFil: Busch, Lucila. Universidad de Buenos Aires. Facultad de Odontología. Cátedra de Farmacología; ArgentinaFil: Sanchez, Gabriel A.. Universidad de Buenos Aires. Facultad de Odontología. Cátedra de Biofísica; Argentin