The purpose of this study was a retrospective survey of 40 patients with infiltrating ductal breast carcinoma to evaluate the relationships between the degree of accumulation of technetium-99m tetrofosmin (Tc-TF), multidrug resistance-related protein (MRP) expression and P-glycoprotein (Pgp) expression in breast cancer tissue. Immunohistochemical analysis (IHA) was performed on pathological specimens of the 40 breast cancers to determine Pgp and MRP expression. The results of IHA, were used as the basis for dividing the 40 breast cancers into four groups: A, 10 tumors with positive MRP and Pgp expressions; B, 10 tumors with positive MRP but negative Pgp expression; C, 10 tumors with negative MRP but positive Pgp expression; and D, 10 tumors with negative MRP and Pgp expression. All 40 patients had undergone Tc-TF mammoscintigraphy to calculate breast cancer uptake of Tc-TF to background uptake (T/B) ratios before IHA and surgery/biopsy. Of the four groups, group A had the lowest T/B ratios (1.15+/-0.10) and group D, the highest (2.19+/-0.15) (P<0.05). The T/B ratios in groups B (1.36+/-0.27) and C (1.37+/-0.26) were intermediate between those of groups A and D. In addition, the T/B ratios were statistically significantly lower in group A than in group B or C, and statistically significantly higher in groups D than in groups B or C (P<0.05). However, no significant difference in T/B ratio was found between groups B and C (P>0.05). Our results indicate that Tc-TF mammoscintigraphy is helpful for in vivo determination of Pgp and MRP expression in breast cancers.