Nijmegen breakage syndrome (NBS) is a rare autosomal-recessive disorder characterized by microcephaly, immunodeficiency and predisposition to cancer, mainly B-cell lymphomas. Our 10-years-old female patient with NBS developed T-cell lymphoblastic leukemia/lymphoma (TLBL/ALL). The use of standard chemotherapy in our patient, except for cranial irradiation, led to complete and sustained remission of TLBL/ALL. In patients with chromosomal instability syndromes chemotherapy must be modified and radiotherapy must be omitted because of potentially serious toxic complications. Careful prevention of infections, including the use of intravenous immunoglobulin is also essential for successful treatment of lymphoid malignancies in NBS. Immunodeficiency in NBS is profound affecting both humoral and cellular immune system. During long-term follow-up after treatment of malignancy our patient remained free of major infections However, the rise of oligoclonal serum IgM was detected recently. Monitoring of serum IgM concentration may be a useful indicator for early detection of lymphomas in NBS.