Nijmegen breakage syndrome (NBS) belongs to the group of chromosomal instability syndromes, is predominantly distributed in Slavic countries and is characterized by microcephaly, combined immunodeficiency and a high predisposition to lymphoid malignancies. A clinical-immunological characteristic of 21 patients with NBS from the Western Region of Ukraine is presented. Cytogenetic disorders were studied in sick children and their parents. The dominating 657del5 mutation of the NBS1 gene is found in all NBS patients in this region. An assessment algorithm for the examination of patients who are suspected of having NBS is proposed, and the main aspects of monitoring and treatment are presented in the article.