Background of the study:Phototherapy is the most widely used form of therapy for unconjugated hyperbilirubinemia. It is a photodynamic stress and can induce lipid peroxidation. Increasing appreciation of the causative role of oxidative injury and lipid peroxidation in the development of many severe diseases of the newborn has lent importance to lipid peroxidation and its possible causes. It converts unconjugated bilirubin to its oxidation products as well as to photo and structural isomers, which are easily eliminated through gastrointestinal tract or lost in urine.In most of our hospitals, phototherapy is given as continuous irradiation only interrupted by short periods of feeding, and light intensity and color of the light being under poor control. Almost all neonates display some level of jaundice, and some display enough serum bilirubin levels that phototherapy or even exchange transfusion is considered.Objective of the study:The present study was designated to assess the effect of phototherapy on total oxidant /antioxidant status in jaundiced fullterm neonates.The study design:Forty fullterm jaundiced newborns were classified into two groups, before phototherapy and the same group after exposure to phototherapy, in addition, 10 age- and sex- matched healthy (non jaundiced) full - term neonates were included to serve as a control group.All the neonates in the study were subjected to full history-taking , clinical examination, routine laboratory investigations including CBC, Blood group and Rh typing for the baby and the mother, Liver function tests (AST&ALT),