X-linked agammaglobulinaemia is a primary immune deficiency, which presents with severe infections early in life. Because of good results of complex therapy, which is based on immune globulin substitution, early diagnosis is imperative, before the development of chronic complications. Due to the inheritance of the disease, it is appropriate to offer genetic counselling and testing to the family members, too. In three čase studies of patients from two families, authors present typical features and effect of therapy. Molecular genetic studies in both of the families were carried out, confirming carriership in the patients' sisters.

        Key words: Bruton's disease, X-linked agammaglobulinaemia, agammaglobulinaemia, IVIG, BTK mutation