BACKGROUND Although endoscopic resection can provide a wide tumor resection with a negative resection margin, it is not yet recommended as a curative therapy for ampulla of Vater cancer. METHODS To investigate the microinvasion rate and the diagnostic accuracy of endoscopic biopsy to properly judge the safety of endoscopic resection for ampulla of Vater cancer. DESIGN Single-center, retrospective study. SETTING Academic medical center. PATIENTS One hundred fifty-nine patients who were finally diagnosed with ampulla of Vater cancer after curative surgical resection. INTERVENTIONS We surveyed the pathologic concordance rate of endoscopic biopsy and the surgical pathology. For the 36 early stage (Tis or T1) cancers, we surveyed the presence of microlymphovascular invasion, gross appearance (intra-ampullary type, periampullary type, or mixed type), and pathologic subtype (intestinal type or pancreaticobiliary type). MAIN OUTCOME MEASUREMENTS Presence of microinvasion in early staged ampulla of Vater cancer. RESULTS Endoscopic biopsy failed to reveal malignancy in 15.9% of the 126 cases. Microlymphovascular invasion was present in 17 cases (56.7%) of the 30 T1 cancers but was absent in all cases of the 6 Tis cancers (P = .02). Neither the gross appearance (P = .51) nor the pathologic subtype (P = .28) could predict the microinvasion rate. LIMITATIONS Single-center, retrospective study with small number of patients. CONCLUSIONS Although endoscopic resection improves the low predictability of endoscopic biopsy, surgical resection should be performed for the T1 stage ampulla of Vater cancer because of the high lymphovascular invasion rate. On the other hand, the safety of endoscopic resection should be evaluated by a large-scale study on Tis cancers to consider the absence of microinvasion.