ABSTRACT This work aimed to Detect Cytotoxic T Lymphocyte-associated antigen 4 (CTLA-4) gene polymorphism in patients with type 1 diabetes mellitus using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) to clarify its role in the susceptibility to type 1 diabetes mellitus. The study was carried out on Forty unrelated Egyptian children with clinically suggestive symptoms and signs of Type 1Diabetes Mellitus (T1DM) (Polyurea , Polydypsia, Polyphagia, history of insulin treatment to control hyperglycemia). Twenty unrelated healthy children matched with patients in age and sex were enrolled in this study as a control group; They had no family history of T1DM or other autoimmune disease. Blood samples were collected from patients and control group aseptically and stored at -20°C until further processing. Typing of cytotoxic T Lymphocyte-associated antigen 4 (CTLA-4)gene polymorphism was done 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