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The T category of distal extrahepatic bile duct carcinoma (DBDC) is based on invasion depth from the basal lamina to the deepest infiltrating tumor cells. Recently, invasive tumor thickness (ITT) was proposed, defined as maximal vertical distance of invasive tumor components regardless of the basal lamina. We compared the predictive value of T category, and ITT grading in 424 surgically resected DBDCs. DBDCs were categorized as 6 Tis (1.4%), 134 T1 (5 mm; 31.6%), 204 T2 (5 to 12 mm; 48.1%), and 80 T3 (12 mm; 18.9%). With ITT, there were 6 G0 (no invasion; 1.4%), 3 G1 (1 mm; 0.7%), 90 G2 (≥1 and5 mm; 21.2%), 188 G3 (≥5 and10 mm; 44.4%), and 137 G4 (≥10 mm; 32.3%). The 5-year survival rates of T1, T2, and T3 were 58.9%, 44.2%, and 18.2%, and those of ITT G1, G2, G3, and G4 were 33.3%, 54.1%, 51.6%, and 26.7%, respectively. The T category discriminated patient survival by overall (P0.001) and pairwise (T1 vs. T2, P=0.007; T2 vs. T3, P0.001) comparisons. ITT grading distinguished survival by overall and between G3-G4 (both P0.001), with no survival differences observed between G1-G2 and G2-G3 comparisons. The T category more accurately discriminated patient survival than ITT grading. To determine the T category for DBDCs, (1) longitudinal sectioning on gross examination, especially for DBDCs with large papillary or nodular growth patterns; (2) evaluation of serial sections or alternative hematoxylin and eosin slides; (3) use of a straight or curved baseline depending on the shape of the peritumoral normal bile duct wall and/or the basal lamina of the peritumoral normal biliary epithelia/biliary intraepithelial neoplasias are recommended.