Tumor staging of extrahepatic bile duct (EBD) carcinoma is problematic for a number of reasons, including definitional problems with the current T classification of the American Joint Committee on Cancer staging system and the common occurrence of severe desmoplastic stromal reaction around the advancing edges of these tumors. To address these problems we evaluated the depth of invasion in 222 cases of EBD carcinoma by measuring the distance between the basal lamina of the adjacent normal epithelium to the most deeply infiltrating tumor cells, and compared this evaluation to time of survival and other clinical and pathologic parameters. A complex pattern of survival time versus the depth of invasion was observed by censored local regression. The recursive-partitioning technique was coupled with the log-rank test to identify 2 significant cutoff points for the depth of invasion, 5 and 12 mm, which segregated patients into 3 groups with statistically significant decreasing length of median survival (<5 mm, 61 mo; 5 to 12 mm, 23 mo; >12 mm, 17 mo, P < 0.001). On the basis of the present data, we propose that a measurement of the depth of invasion should be performed in cases of EBD carcinoma, and that the T classification of EBD carcinoma should be changed to incorporate this measurement: T1 (<5 mm), T2 (5 to 12 mm), and T3 (>12 mm).