DNA double-strand break (DSB) is one of the most deleterious lesions induced by DNA damaging agents. DSB repair pathway is implicated in maintaining genomic integrity via suppression of genetic instability and neoplastic transformation. DNA-dependent protein kinase (DNA-PK) has a pivotal role in DNA DSB repair. The Nijmegen breakage syndrome protein (NBS1), essential for DSB repair, re-localizes into subnuclear structures upon induction of DNA damage by ionizing radiation, forming so-called ionizing radiation-induced foci (IRIF), which is visualized by immunostaining. We measured DNA-PK activity and the number of persistent NBS1 IRIF per nucleus 24 h after irradiation of peripheral blood lymphocytes (PBL) from patients with sporadic breast cancer. Chromosomal aberrations were examined by cytogenetic methods. We examined the relationship between these measurements and clinical characteristics of patients such as tumor size, lymph node metastasis and nuclear grade of cancer cells. A higher number of NBS1 IRIF or lower DNA-PK activity correlated with higher chromosome instability. Patients whose PBL had lower DNA-PK or higher NBS1 IRIF had aggressive cancer phenotypes such as a larger tumor, higher nuclear grade and positive axillary lymph node metastasis. The combination of DNA-PK activity and NBS1 IRIF were useful for predicting lymph node metastasis. The ability of DSB repair in PBL is related to aggressive breast cancer phenotypes. Axillary lymph node dissection can be avoided by examining DNA-PK activity and NBS1 IRIF of PBL, which can contribute to improving the quality of life of breast cancer patients.