In a study comprised of 53 breast cancer patients, chosen for determining the cytogenetic damage to peripheral blood lymphocytes (PBL) due to cancer therapy, eleven patients revealed chromosomal abnormalities in 3.18% of cells prior to any such therapy. Age matched controls showed 0.18% abnormal cells. Statistical analysis of the abnormal cells in patients prior to therapy and normal controls revealed significant values at 1% level (p < 0.01). The factors causing "chromosome instability" causing chromosome breakage or whether any clastogenic metabolites produced by the tumor itself is triggering neoplastic transformation are discussed.