Forty patients with breast mass and 10 female controls underwent double-phase (15 min and 3 h) 99Tcm-sestamibi (MIBI) imaging. Another four post-mastectomy patients with suspected metastases underwent total body imaging at 15 min. The 99Tcm-MIBI images of all 10 female controls showed symmetrical homogeneous radioactivity in both breasts. A focal area of increased 99Tcm-MIBI uptake in the breast tumour is considered as positive. Eleven breasts of 10 patients underwent total mastectomies with lymph node dissection, with one patient having bilateral mastectomy. These 11 breast tumours were confirmed to be infiltrating ductal carcinoma. All 11 breast carcinomas had positive early and delayed phase results with the exception of one patient who had only a focal increased uptake in early phase which happened to be the smallest size of carcinoma. Focal areas of positive MIBI uptake in the axillae in 4 of 10 patients were confirmed to be metastatic carcinoma of the breast. One patient with breast carcinoma underwent mastectomy whose 99Tcm-MIBI imaging was negative. Twelve patients undergoing excisional biopsy or lumpectomy of breast resulted in 7 fibroadenomas, 2 papillomas, 1 abscess, 1 granuloma and 1 chronic inflammation. Four of 7 fibroadenomas having hypercellularity histopathologically exhibited a focal MIBI uptake on double-phase imaging. One granulomatous disease had positive early phase but faded on the delayed image. The patients with absence of MIBI uptake included 3 fibroadenomas, 2 papillomas, 1 abscess, 1 chronic inflammation and 17 fibrous hyperplasia.(ABSTRACT TRUNCATED AT 250 WORDS)