INTRODUCTION: ^99mTc-Sestamibi (MIBI) scintimammography is a functional imaging technique that may be useful in monitoring and predicting the response of breast cancer to neoadjuvant chemotherapy. The aim of this thesis was to examine MIBI pharmacokinetics and investigate key parameters for their ability to monitor and predict the response of breast tumours to chemotherapy. METHOD: Patients with breast cancer were imaged with MIBI before and after chemotherapy. Chemotherapy response was assessed clinically, ultrasonographically and histologically. MIBI uptake was compared with chemotherapy response to determine if it could predict or monitor response. Phantom studies were undertaken to determine the optimum method of quantifying retention, washout and efflux of MIBI out of the tumour. The efflux rate of MIBI was compared with chemotherapy response to determine if it could predict response. RESULTS: 25/26 patients showed a reduction in MIBI uptake post-chemotherapy. 4 patients demonstrated uptake post-chemotherapy and all had significant residual disease. 22 patients had negative scintimammograms post-chemotherapy although 3 of these patients had large tumours on histology. 10/40 patients with uptake values >2.0 pre-chemotherapy had good final outcomes. 30/40 patients had uptake values <2.0 pre-chemotherapy of which 26 had good outcomes and 4 had poor outcomes. The most accurate method of measuring retention, washout and efflux rate was to use background subtracted counts. Efflux rate measurements were the most reproducible. No statistically significant difference in pre-chemotherapy efflux rates between responders and non-responders was demonstrated. An efflux rate > 6.0 x 10^-3 minute^-1 predicted a poor response to chemotherapy ultrasonographically. CONCLUSION: MIBI uptake can be used to monitor, and also predict good, chemotherapy response. MIBI uptake cannot predict a poor chemotherapy response.