Some researchers tried to use Tc-99m MIBI scintigraphy for evaluating breast cancer patients. Early reports revealed that TC-99m MIBI breast scintigraphy is useful in distinguishing malignancies from benign masses in patients who have difficulty in mammographic evaluation. Further studies suggested that the functional imaging with Tc-99m MIBI in breast cancer seems to be correlated with levels of P-glycoprotein (Pgp) expression and angiogenesis in cancer tissues. So we evaluated whether there is an actually significant relationship between the pattern of Tc-99m MIBI uptake in tumor tissue and tumoric factors including Pgp expression and angiogenesis. Thirty one untreated breast cancer patients (pathologically proved to invasive ductal carcinoma later) were prospectively studied by both Tc-99m MIBI scintigraphy and immunohistochemical analysis of angiogenesis and Pgp expression on sugically removed tumor tissues. Both lateral and anterior planar images were obtained at 10 minutes and 3 hours after intravenous injection of 740 MBq Tc-99m MIBI. Tumor-to-normal breast ratio (T/N) and washout Index (WI, early T/N minus late T/N divided by early T/N) were calcuated. The results were follows; A significant correlation was found between T/N at early and late images (r=0.54, p=0.002 on early images, r=0.47, p=0.008 on late images). T/N of early images were not different among different groups of Pgp expression (p=0.3696), while those of late images were significantly different among groups (p=0.0276). Even more significant difference of WI among the groups were noted (p=0.0015). A significant negative correlation was noted between T/N of late images and Pgp expression (p=0.00276). A even stronger correlation was found between WI and Pgp expression (r=0.68, p=0.00l). In conclusion, the tumoral uptake and washout of Tc-99m MIBI can be used as a simple functional test for angiogenesis and P-glycoprotein expression in untreated breast cancer.