Breast tumors harbor extensive intratumoral heterogeneity (ITH), both within primary and metastatic lesions. The generation of this genetic diversity relies on chromosomal instability (CIN), a dynamic and complex multilayered phenotype. Centrosome amplification (CA), a well-established cancer cell-specific trait is known to compromise mitotic fidelity resulting in CIN. We hypothesized that CA is profoundly crucial in driving ITH associated with tumor progression. While some studies suggest that CA is an indicator of aggressiveness, others report that extent of CA increases with grade. No study yet has yet quantified CA or emphatically demonstrated the extent of CA during the progression of a well-differentiated to a poorly-differentiated tumor. We have developed first-of-a-kind novel method to quantitate the degree, extent and type of CA within breast tumor samples across grades. Tissue specimens from 200 breast tumors were immunostained for centrosomes and nuclei. Employingconfocal imaging, a stack of optical sections was acquired i within 10 regions of interest (ROIs) per sample. Centrosomes were categorized as (i) individually-distinguishable centrosomes (iCTRs) or (ii) megacentrosomes (mCTRs) comprised of several tightly clustered centrosomes. For each ROI, number of nuclei as well as numbers and volumes of iCTRs and mCTRs were determined and a cumulative Centrosome Amplification Score (CAS) was obtained as CAStotal = CASi+CASm. Low-grade (n=75) tumors exhibited significantly higher CASi (3.9 vs 2.3), CASm (9.5 vs 5.4) and CAStotal (12.8 vs 8.05) than high-grade (n=125) ones, which disapproves the previously held notion that CA increases during disease progression. This postulation is supported by the observation that low-grade tumors exhibiting lymph node infiltration and metastasis (n=30) had higher CASmas compared to the non-invasive ones (n=50). In conclusion, we have established CA as a “quantifiable cell-biological property”, that can potentially predict the risk of a low-grade tumor being or becoming an aggressive and invasive one.