The purpose of this study was to assess the contribution of techentium-99m methoxyisobutylisonitrile (Tc-99m sestamibi) scintimammography in the diagnosis of breast cancer in 32 female patients with indeterminate mammographic probability of malignancy because of mammographically dense breasts. All the breast masses were removed and histopathological diagnoses were obtained in all cases. The results showed that scintimammography with Tc-99m sestamibi was positive in 21 patients (20 true-positive, 4 false-positive) and negative in 11 patients (7 true-negative, 4 false-negative). The diagnostic sensitivity, specificity and accuracy rates were 83%, 88% and 84%, respectively, in the differentiation of malignant from benign breast masses in the patients with mammographically dense breasts. The detection of malignant breast tumors by Tc-99m sestamibi scintimammography was independent of the density of the breast tissue. In conclusion, Tc-99m sestamibi scintimammography appears to be a useful diagnostic method for the detection of breast cancer in patients with non-diagnostic mammograms because of mammographically dense breasts.