PURPOSE: The objective of our study was to compare the value of differential diagnosis of ultrasonography with power Doppler imaging(PDI) and 99mTc-MIBI scintimammography in the evaluation of breast lesions. MATERIALS AND METHODS: Fifty-five breast lesions in 48 patients underwent both 99mTc-MIBI scintimammography and sonography with PDI. Lesions were classified as benign or malignant on the basis of the absence or presence of a focus of increased activity at scintimammography, and the shape, border, echogenicity and amount of flow at sonography. The accuracy of the two modalities according to tumor size was compared. RESULTS: Pathologic diagnosis showed that among the 55 lesions, 40 were malignant and 15 were benign. In the detection of breast cancer, sensitivities and specificities were 97.5%(39/40) and 66.7%(10/15) for ultrasonography with PDI and 72.5%(29/40) and 86.7%(13/15) for scintimammography. For 32 lesions smaller than 2 cm, sensitivities and specificities were 94.7%(18/19) and 61.5%(8/13) for ultrasonography with PDI and 57.9%(11/19) and 84.6%(11/13) for scintimammography. For 23 lesions 2 cm or larger, these parameters were 100%(21/21) and 100%(2/2) for ultrasonography with PDI and 85.7%(18/21) and 100%(2/2) for scintimammography. CONCLUSION: In the diagnosis of breast cancer, ultrasonography with PDI was less specific than 99mTc-MIBI scintimammography. Ultrasonography with PDI was, however, more sensitive than 99mTc-MIBI scintimammography, particularly for lesions smaller than 2 cm.