Background: Children with Down syndrome appear to be more likely susceptible to respiratory tract infections and it is an important cause of morbidity and mortality among them. Aim of the study: It was to investigate the relationship between levels of IgG and IgM in Down syndrome and recurrent lower respiratory tract infections. Subjects and Methods: This study was conducted on 40 children who were categorized into 2 groups: patient group, which includes children with Down syndrome patients presented with LRTIs, their karyotyping revealed trisomy 21 in all cases and their age range from 1 to 13 years. Control group, which includes 20 healthy children matched for sex and age with patient group. They were subjected to full history taking & thorough clinical examination, and they investigated for complete blood picture, C-reactive protein and serum immunoglobulins (IgG & IgM) using ELISA technique. Results: Plasma levels of immunoglobulin G (IgG) were elevated in Down syndrome with significant increase the frequency of lower respiratory tract infections and hospital admission. Plasma levels of immunoglobulin M (IgM) were decrease in down patients with significant increase in the frequency of lower respiratory tract infections and hospital admission. Conclusion: These findings suggest the importance of estimation and follow up of serum levels of IgG &Ig M in cases of Ds with recurrent respiratory tract infections.