The hyper-IgM (HIGM) syndromes are a group of molecular defects of immunoglobulins class switch during B cell maturation. In HIGMs the level of IgM is elevated in majority of cases but also can by in normal range or even decreased. The other class of immunoglobulins are decreased or absent. The five types of HIGM are known: one X-linked (T lymphocyte defect) and four (including B cell defects) transmitted in the autosomal recessive pattern. The 4 cases of HIGM are shown: 3 boys diagnosed at age of 1, 6 and 15 years and one girl with DiGeorge syndrome and HIGM developed at age 4. In 2 cases the haematological symptoms (thrombocytopenia and neutropenia) were observed. The expression of CD40L on activated T lymphocytes was present in all patients but the level of expression was different. The low level of CD40L was associated with early onset and severe course of infections. All patients are treated with intravenous IgG substitution what resulted in decrease the frequency and severity of infections.