Purpose: The purpose of this study was to evaluate the diagnostic usefulness of scintimammography performed with Tc-99m tetrofosmin in the detection of primary breast cancer. Materials and Methods: Sixty-one patients underwent Tc-99m tetrofosmin scintimammography, plain-filrn mammography and ultrasonography. After intravenous injection of Tc-99m tetrofosmin (740 MBq), prone lateral and anterior scintimammograms were obtained. Scintimammogram was visually interpreted as positive, probably positive, probably negative and negative for malignancy. The tumor to background count ratio (T/B) was measured at 5 minutes and 1 hour. Plain-film mammogram was interprcted as one of 5 categories. Final diagnosis was achieved by surgical histology (58/61) or fine needle aspiration (3/61). Of 61 patients, 44 had cancer and 17 had benign lesion. Tumor size of malignant and benign lesions on ultrasonogram were 2.51+-1.30 cm (range 1-8 cm), 2.50+-1.35 cm (range 0.96-6 cm), respectively. Results: The sensitivity of plain-film mammography was 88.6%, specificity 58.8%, positive predictive value 84.7%, and negative predictive value 66.7%. The sensitivity of Tc-99m tetrofosmin scintimammography was 90.9%; specificity, 88.2%; positive predictive value, 94.9%; negative predictive value, 78.9%. Of 25 patients with indeterminate degree of suspicion for malignancy on plain-film mammogram, 23 were correctly diagnosed by scintimamography. The T/B at 5 minutes and 1 hour were 3.78+-2.21, 3.25+-1.80 respectively. The T/B was decreased significantly at 1 hour (p<0.001). Conclusion: Tc-99rn tetrofosrnin scintimammography was useful diagnostic procedure in the detection of primary breast cancer, especially in patients with indeterminate degree of suspicion for malignancy on plain-film mammogram. (Korean J Nucl Med 1999;3.3:452- 460)