X-Linked Agammaglobulinemia (XLA) is the major primary immunodeficiency in which the body is unable to produce the antibodies responsible for the defense against bacteria and viruses. The patient (a 6-month-old boy) was born at term to non-consanguineous parents. Both parents and older sister are clinically healthy. At 3, 5 months of age he presented acute viral infection with glue ear. At 4 month-serous meningitis, at 5 month-laryngotracheitis and serous meningitis. The levels of immunoglobulins were decreased for Ig A and IgG isotypes. The virtual lack of CD 19+ B-lymphocytes was defined. Additionally there was found a complete absence of KREC (kappa-deleting recombination excision circle) in dried blood spot. The molecular diagnostics of coding region of the BTK gene was performed. DNA sequencing analysis of patient showed a 13-bp deletion in exon 2 (c.64_76delCCTCTAAACTTCA), leading to occurrence of frameshift and premature termination codon (p.Pro22fsTer28). This mutation was not described earlier. The mother and the sister of proband showed heterozygosity at the same position. Prenatal diagnostic testing has become available to this family for next pregnancy.