Background: Neonatal hyperbilirubinemia is one of the most common caused for NICU admission and phototherapy is the main line of treatment in most cases with indirect hyperbilirubinemia either in fullterm or preterm neonates in NICU. The effect of phototherapy on the peripheral blood elements is still under evaluation.
Objective: To evaluate the effect of phototherapy used in treatment of neonatal indirect hyperbilirubinemia on the number of blood eosinophils.
Patients and Methods: This prospective case control study included 100 neonates with indirect hyperbilirubinemia treated with phototherapy in NICU. group 1 included 25 fullterm neonates treated with phototherapy while group 2 included 25 preterm neonates treated with phototherapy and group 3 and group 4 included 25 full term and 25 preterm with neonatal indirect hyperbilirubinemia who were not treated by phototherapy. 
Results: Regarding eosinophils count, In full term neonates (group 1), there was no significant difference in eosinophils count after 24 hours compared to its level at admission (p>0.05) while in preterm neonates (group 2), there was significant increase in eosinophils levels after 24 hours compared to its levels at admission (p=0.020). Regarding lymphocytes, there was a significant decline in lymphocytes level after 24 hours compared its level at admission (p=0.001) in both fullterm and preterm neonates.
Conclusion: Increase in the level of peripheral eosinophils can occur in preterm neonates with neonatal hyperbilirubinemia treated by phototherapy while there was no significant difference in eosinophilic count after 24 hours after stoppage of phototherapy compared to its levels at admission in full-term neonates. This Increased eosinophilic count may predispose to allergic disease in the future.