The aim of our study was to compare the usefulness of technetium-99m tetrofosmin (Tc-TF) mammoscintigraphy and ultrasonography for detecting breast cancer in mammographically dense breasts. This study included 32 female Taiwanese patients with indeterminate mammographic probability of malignancy due to mammographically dense breasts. Both Tc-TF mammoscintigraphy and ultrasonography were performed on each patient. Then, all of the 32 breast masses underwent biopsies or operations to obtain final pathological diagnoses. Based on the final pathological diagnoses, 24 women had breast cancer and eight had benign breast tumors among the 32 patients. Twenty cancers and one benign tumor had positive Tc-TF mammoscintigraphic findings. The sensitivity, specificity, and accuracy were 83, 88, and 84%, respectively. Twenty-two cancers and five benign tumors had positive ultrasonographic findings. The sensitivity, specificity, and accuracy were 92, 38, and 78%, respectively. To detect breast cancer in patients with nondiagnostic mammograms because of mammographically dense breasts, ultrasonography with its higher sensitivity is more suitable for screening breast masses. However, due to its higher specificity, Tc-TF mammoscintigraphy is useful to confirm the ultrasonographic findings.