Type 1 diabetes is one of the most common chronic childhood illnesses. Interplay between genetic susceptibility and environmental factors is thought to provide the fundamental element for the disease. It has been shown that more than 40 genetic loci are associated with T1DM. Important one among these is the CTLA-4. This work aimed to detect Cytotoxic T Lymphocyte-associated antigen 4 (CTLA-4) gene polymorphism in patients with type 1 diabetes mellitus T1DM using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) to clarify its role in the susceptibility to T1DM. The study was carried out on forty unrelated Egyptian children with TIDM. Twenty unrelated healthy children were enrolled as a control group. Blood samples were collected from patients and control groups and subjected to CTLA-4 gene polymorphism analysis using polymerase chain reaction followed by restriction fragment length polymorphism (PCR-RFLP). CTLA-4 G allele and GG homozygous genotype were significantly increased in T1DM patients than in control group (P