Background: Rebound hyperbilirubinemia is the return of total serum bilirubin to phototherapy threshold within 72 hours of phototherapy termination. Objective: to determine the prevalence and magnitude of post-phototherapy bilirubin rebound needing reinstitution of phototherapy.pateints and Methods: observational cross-sectional study in NICU department of Aswan University Hospital from January 2021 to December 2021.Results: It included 101 neonates with hyperbilirubinemia at age of 71.5 ± 46.1 hours, 50 male, and 51 females. The most common cause of jaundice is ABO incompatibility in 58.4% of cases. Exchange transfusion was done to 16% of cases and blood transfusion to 6%. The incidence of rebound is 12.9%. The Mean (95% CI) bilirubin after 24 hours was 8.71 [8.27- 9.14], and after 72 hours were 6.82 [5.77- 7.87]. There were statistically significant differences between the rebound and non- rebound neonate in term of hemoglobin, MCH and total bilirubin after 72 hours. The results showed highly statistically significant effect of blood transfusion on rebound occurrence. Our findings suggest that if the cause of the jaundice was hemolytic, then it is more likely to develop rebound hyperbilirubinemia. Conclusion: the prevalence of rebound hyperbilirubinemia is high .so repeated follow up after discharge is mandatory