Abstract: Neoadjuvant chemotherapy(NCT) has become a standard therapeutic method for focal advanced breast carcinoma. The multidrug resistance(MDR) of tumors is one of the main causes of chemotherapy failure in breast cancer. MDR development involves the transmembrane P-glycoprotein(P-gp) encoded by the MDR gene MDR1. ⁹⁹Tc^m-methoxyisobutylisonitrile(MIBI) is a radiotracer for scintigraphy of tumors. MIBI is also a transport substrate for P-gp and has been used in assessing P-gp-mediated MDR in a variety of tumors. The nuclear medicine community in the USA has suggested the use of ⁹⁹Tc^m-MIBI scintigraphy as a noninvasive method in in vivo imaging of MDR. Studies have shown that early imaging of ⁹⁹Tc^m-MIBI reveals the accumulative quantity of MIBI in breast tumors, and correlated with malignant extent of tumor. Thus, early imaging through ⁹⁹Tc^m-MIBI can be conducted to evaluate the curative effect of NCT in breast cancer. Delayed imaging reveals the washout rates of MIBI from breast tumors, which is correlated with P-gp expression and MDR. Thus, delayed ⁹⁹Tc^m-MIBI may provide important information on P-gp-mediated MDR and may predict chemotherapy sensitivity.