Cholangiocarcinomas are topographically categorized as intrahepatic or extrahepatic by the International Classification of Diseases for Oncology (ICD-O). Although hilar cholangiocarcinomas (Klatskin tumors) are extrahepatic cholangiocarcinomas, the second edition of the ICD-O (ICD-O-2) assigned them a histology code 8162/3, Klatskin, which was cross-referenced to intrahepatic cholangiocarcinoma. Recent studies in the United States that included this code (8162/3, Klatskin) with intrahepatic cholangiocarcinoma reported an increasing incidence of intrahepatic cholangiocarcinoma and a decreasing incidence of extrahepatic cholangiocarcinoma. To investigate the impact of this misclassification on site-specific cholangiocarcinoma incidence rates, we calculated annual percent changes (APCs) with data from the Surveillance, Epidemiology, and End Results (SEER) program using a Poisson regression model that was age-adjusted to the year 2000 U.S. population. All statistical tests were two-sided. During 1992-2000, when SEER used ICD-O-2, 1710 intrahepatic cholangiocarcinomas, 1371 extrahepatic cholangiocarcinomas, and 269 hilar cholangiocarcinomas identified by code 8162/3, Klatskin were diagnosed. Ninety-one percent (246 of 269) of the hilar cholangiocarcinomas were incorrectly coded as intrahepatic cholangiocarcinomas, resulting in an overestimation of intrahepatic cholangiocarcinoma incidence by 13% and underestimation of extrahepatic cholangiocarcinomas incidence by 15%. However, even after the exclusion of tumors that were coded to the histology code 8162/3, Klatskin, age-adjusted annual intrahepatic cholangiocarcinoma incidence increased during this period (APC = 4%, 95% confidence interval = 2% to 6%, P<.001).