18q deletion syndrome (OMIM #601808) results from a deletion of a part of a long arm of 18 chromosome and is characterized by mental retardation and congenital malformations. We present an exceptional case of a 12-year-old girl with severe phenotype of 18q deletion syndrome, frequent infections, type 1 diabetes, autoimmune thyroiditis, and vitiligo. At first, the patient was diagnosed with selective immunoglobulin A (sIgAD) which explained her susceptibility to both infections and autoimmunity. With time, sIgAD progressed to common variable immune deficiency-like (CVID-like) disorder. She had a minimum of 12 infections per year, approximately twice as many courses of different antibiotics and up to three hospitalizations annually, making the treatment of diabetes difficult. Due to safety issues (increased risk of adverse reaction to blood products) and patient's convenience, subcutaneous IgG (SCIG) replacement therapy was initiated. We noticed a substantial decrease in the number of infections and improvement of metabolic control of diabetes.