We evaluated the efficacy of 99Tcm-sestamibi (MIBI) scintimammography for the detection of breast cancer in 332 patients. Two hundred and seven scans were confirmed by histological or cytological results; the other patients were examined because they belonged to high-risk groups or had dense fibroglandular breasts. Of 207 patients with histological confirmation, 112 positive studies were obtained: 86 true- positive and 26 false-positive. Scintimammography was negative in 95 patients: 88 true-negative and seven false-negative. Six of seven false-negative results were obtained in patients with impalpable tumours. The sensitivity, specificity, positive and negative predictive values were 92.5%, 77.2%, 76.8% and 92.6% respectively. The overall accuracy was 84.1%. To identify false-positive results, the count ratio of the target lesion to the contralateral normal area on 38 true-positive scans and in 26 false-positive examinations was calculated from the region of interest drawn on the 99Tcm-MIBI scan (L/N ratio). A significantly higher ratio was found for the true-positive scans (1.583 ± 0.501 vs 1.246 ± 0.213; P = 0.0002). In conclusion, 99Tcm-MIBI scintimammography is a sensitive and accurate method for the detection of breast malignancies.