Technetium-99m sestamibi scintimammography has been used primarily in the diagnosis of breast cancer. It has also been suggested that this technique could be used to monitor response to chemotherapy and possibly to predict those patients in whom no response can be expected. An initial study was performed in nine patients with primary breast cancer. All patients underwent prone lateral and anterior 99mTc-sestamibi imaging at diagnosis and 4-7 months later, after they had received cytotoxic chemotherapy. The uptake of 99mTc-sestamibi in the breast was compared with that in normal surrounding breast tissue and this ratio was expressed as the target to background ratio. In all patients treated there was a reduction in uptake of 99mTc-sestamibi after treatment, such that whilst all the tumours could be seen before treatment, only three were visible following chemotherapy. There was a significant fall in the mean target to background ratio of the patients undergoing chemotherapy: the tumour to background ratio was 2.48 before chemotherapy and 1.40 after treatment (P<0.001, paired Student's t test). This fall in tumour activity was observed both in those patients in whom a clinical response was seen and in the two patients in whom the tumour enlarged despite chemotherapy. It appears that the reduced uptake of 99mTc-sestamibi seen after chemotherapy may be a non-specific change and therefore may not be predictive of the clinical response to treatment.