We studied 30 patients with infiltrating ductal breast carcinomas to evaluate the relationship between the degree of accumulation of technetium-99m tetrofosmin (Tc-TETRO) and p-glycoprotein (Pgp) expression in breast tumor tissues. All of the 30 patients underwent Tc-TETRO mammoscintigraphy to calculate breast tumor uptake of Tc-TETRO to background (T/B) ratios before surgery or biopsy. Immunohistochemical analysis was performed to determine Pgp expression in the pathological specimens of the 30 breast tumors. The T/B ratios were significantly lower for tumors in 12 patients with positive Pgp expression (Group A) than for those in 18 patients with negative expression (Group B) (1.20 +/- 0.12 and 1.94 +/- 0.30, p < 0.05). Our results supported the opinion that Tc-TETRO mammoscintigraphy is helpful for determining in vivo the presence of multidrug resistance due to Pgp expression in breast carcinoma patients.