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To evaluate the diagnostic accuracy of esophageal and cardia adenocarcinoma in the Swedish Cancer Register.Based on cancer registers, a rising incidence of esophageal and cardia adenocarcinoma has been reported in several populations, but possible influence of differences in tumor classification has not been evaluated.In a nationwide study in 1995 through 1997, all Swedish patients, born in Sweden and younger than 80 years with esophageal or cardia adenocarcinoma and half of all patients with esophageal squamous cell carcinoma, were prospectively, uniformly, and thoroughly classified. This study classification was compared with the tumor classification in the Swedish Cancer Register, which is based on routine clinical practice.The overall completeness of the Cancer Register was high (98.3%), whereas the site-specific completeness of the Register was 63% for esophageal adenocarcinoma, 74% for cardia adenocarcinoma, and 91% for esophageal squamous cell carcinoma. The incidence of esophageal adenocarcinomas was 16% higher in the study classification compared with that of the Register during the study period, whereas the incidence of cardia adenocarcinoma was 2% lower in the study classification.There is a diagnostic mismatch between esophageal and cardia adenocarcinoma in the clinical setting and, therefore, also in Cancer Registers. In etiologic and therapeutic research, this problem needs consideration, since these tumors have distinct risk factor profiles and could be subjected to different treatment strategies. The increasing incidence rate of esophageal adenocarcinoma in Sweden is unlikely to be explained by such differences in tumor classification, however.