Background/objectives: hyper-IgM (HIGM) syndrome is characterized by normal to increased serum IgM, as well as very low or undetectable IgG, IgA, and IgE. HIGM (also known as class-switch recombination (CSR) defects) patients indicate different clinical manifestations such as autoimmune disorders. The present study aimed to evaluate demographic data, clinical manifestation, and immunological findings in HIGM patients. Methods: Clinical features and immunological data were collected from medical records belonged to the 79 Iranian HIGM patients diagnosed in Children’ s Medical Center in Iran. To compare clinical records and laboratory data, all HIGM patients were classified into two different groups as follows: patients with autoimmune disease and patients without autoimmune diseases. Results: A total of 79 patients (60 male and 19 female) with median (IQR) age of 12 years old at the time of the study were enrolled (6-22. 45). Autoimmunity diseases were seen in 19 patients (23. 75%, 3 females and 16 males). Among the noninfectious manifestations, the hepatomegaly and spelenomegaly were significantly higher in the patients with autoimmunity (p= 0. 006), compared to the patients without autoimmunity (p=0. 006). The most common autoimmune presentations among HIGM patients were ITP (32%), juvenile rheumatoid arthritis (16%), autoimmune hemolytic anemia (11%), Sclerosing cholangitis (11%), Gullain-Barré syndrome, Evans syndrome, diabetes mellitus, and chrohn’ s disease. Conclusions: The relationship between HIGM syndrome and autoimmunity disorders could lead to sever clinical complications. Therefore, we suggested that immunologists should be aware of this complications.