Purpose: The aim of this study was to investigate the diagnostic role of 99mTc-Tetrofosmin in detection of breast cancer and compared with that of 99mTc-MIBI. Material and Methods: Forty-eight patients with a clinically palpable mass or abnormal mammographic or ultrasonographic findings had 99mTc-MIBI and 99mTc-Tetrofosmin scintimammographies after intravenous injection of 925 MBq of radiopharmaceuticals. The scintimammographs were correlated with histopathologic findings. Results: Thirty-three patients were diagnosed with breast cancer and 15 patients with benign breast diseases. The numbers of true positive, true negative, false positive, and false negative cases of 99mTc-MIBI scintimammography were 29, 10, 5, and 4 respectively. The sensitivity, specificity, positive predictive value, and negative predictive value of 99mTc-MIBI scintimammographies were 87.8%, 66.7%, 85.3%, and 71.4% respectively. The numbers of true positive, true negative, false positive, and false negative cases of 99mTc-Tetrofosmin were 31,10, 5, and 2 respectively. The sensitivity, specificity, positive predictive value, negative predictive value of 99mTc-Tetrofosmin were 93.9%, 66.7%, 86.1%, and 73.3% respectively. One patient was false negative in both 99mTc-MIBI and 99mTc-Tetrofosmin scintimammographies and its size was 0.5㎝. Conclusion: 99mTc- Tetrofosmin and 99mTc-MIBI were non-invasive and useful in detection of breast cancer and 99mTc- Tetrofosmin was comparable to the 99mTc-MIBI in detection of primary breast cancer. (Korean J Nucl Med 2000;34:119-128)