Background/Purpose: Anomalous arrangement of the pancreaticobiliary duct (AAPBD) is a malformation of the bile and pancreatic ducts which is commonly associated with congenital biliary dilatation and predominantly occurs in girls. Estrogen receptor (ER) is reported to modulate cholangiocyte proliferation and play a role in tumorigenesis of estrogen-dependent malignancies. The present study investigated the presence of ER in the gallbladder of patients with AAPBD to elucidate whether ER expression is correlated with sexual specificity and biliary histology. Methods: Specimens comprised samples of 29 gallbladders from patients with AAPBD (21 girls and 8 boys;mean age at surgery, 5.3 years). Medical records were retrospectively reviewed, and sex, age, radiographic parameters, and laboratory data were collected for this study. Immunohistochemistry was performed using anti-ER and anti-MIB- 1 antibodies, and MIB- 1 score was calculated for evaluation of cell proliferation. Results: ER expression was observed in both the nuclei and cytoplasm of gallbladder epithelium in 12 of 29 specimens. No difference in background characteristics were noted between ER-positive and ER-negative groups. Mucosal hyperplasia was found in 17 specimens. ER-positive cases were significantly more common in AAP-BD with mucosal hyperplasia than without (P < 0.03). MIB- 1 score did not differ significantly between ER-posi-tive and ER-negative groups. Conclusions: ER expression seems to be related to hyperplastic mucosa of the gallbladder in AAPBD. ER might play a role in the proliferation of gallbladder epithelium. Background / Purpose: Anomalous arrangement of the pancreaticobiliary duct (AAPBD) is a malformation of the bile and pancreatic ducts which is commonly associated with congenital biliary dilatation and predominantly occurs in girls. Estrogen receptor (ER) is reported to modulate cholangiocyte proliferation and play a role in tumorigenesis of estrogen-dependent malignancies. The present study investigating the presence of ER in the gallbladder of patients with AAPBD to elucidate whether ER expression is correlated with sexual specificity and biliary histology. Methods: Specimens formed samples of 29 gallbladders from patients with AAPBD (21 girls and 8 boys; mean age at surgery, 5.3 years). Medical records were retrospectively reviewed, and sex, age, radiographic parameters, and laboratory data were collected for this study. Immunohistochemistry was performed using anti-ER and anti-MIB - 1 antibodies, and MIB- 1 score was calculated for evaluation of cell proliferation. Results: ER expres sion was observed in both the nuclei and cytoplasm of gallbladder epithelium in 12 specimens of 29 specimens. No difference in background characteristics were observed between ER-positive and ER-negative groups. common in AAP-BD with mucosal hyperplasia than without (P <0.03). MIB-1 score did not differ significantly between ER-posi-tive and ER-negative groups. Conclusions: ER expression seems to be related to hyperplastic mucosa of the gallbladder in AAPBD. ER might play a role in the proliferation of gallbladder epithelium.