The potential of Tc-99m tetrofosmin for the imaging of breast carcinoma and axillary lymph node metastases was investigated and compared with that of Tc-99m MIBI. Thirty female patients with palpable breast masses underwent Tc-99m MIBI scintigraphy; 17 of those underwent Tc-99m tetrofosmin scintigraphy. The axillary and breast regions were evaluated in all patients. All patients underwent biopsy within 2 weeks of the study. Twenty patients were found to have a primary malignancy of the breast, whereas 10 had benign disease. The patients with breast carcinoma had surgery. Twelve patients had axillary lymph node metastases. Tc-99m MIBI breast imaging showed abnormal uptake in 18 of 20 malignancies and in 8 of 12 axillary lymph node metastases. Tc-99m tetrofosmin breast imaging showed abnormal uptake in 13 of 14 malignancies and in 6 of 10 axillary lymph node metastases. Sensitivity, specificity, and accuracy values obtained with Tc-99m MIBI and Tc-99m tetrofosmin scintigraphy for breast carcinoma were 90%, 90%, 90%, and 93%, 100%, 94%, respectively. The values obtained with Tc-99m MIBI and Tc-99m tetrofosmin scintigraphy for axillary lymph node metastases were 66%, 100%, 86%, and 60%, 100%, 76%, respectively. The authors conclude that both of these techniques are effective in the differentiation of malignant breast masses from benign ones and in detecting axillary lymph node metastases. However, Tc-99m tetrofosmin is superior to Tc-99m MIBI in detecting breast carcinoma.