AIM: To evaluate any differences between the percentages of involved breast volume, pathologic attributes,and tumor marker expression of T3 and T4a-c tumors in locally advanced breast cancers(BC).METHODS: All patients with T3 N > 0 and T4a-c BC without evidence of distant metastasis(M0), presenting to the Breast Clinic from 1980 to 2010, were examinedto determine whether their BC’s involved ≥ 50% of their breast volumes, defined by gross replacement of at least one hemisphere. Core needle biopsy or postmastectomy specimens from tumors involving a known percent of breast volume were evaluated for:(1)pathological grades and lympho-vascular invasion(LVI);(2) hormone receptor(ER/PR) expression > 0; and(3)epidermoid growth factor 2(her2) over-expression(3+)by immune-histochemical staining or fluorescent in situ hybridization.RESULTS: The data base included 98 patients with T3N> 0 M0 and 120 with T4a-c, any N disease, M0 disease. T3 tumor masses involved 50% or more of the breast in 23/98(24%), and T4a-c tumors 65/120(54%)(P < 0.001). Only 1% of T3 tumors and 23% of T4a-c tumors presented with total breast replacement. There were no significant differences between the pathological attributes and marker expression of the T3 and T4a-c tumors.CONCLUSION: These data suggest that erosion of the overlying skin or underlying chest wall by some BC may be due to neglect and delay, rather than inherent biological aggressiveness.