Accurate diagnosis of dysplasia and adenocarcinoma in patients with Barrett esophagus is critical for clinical decision-making and patient management. IMP3 is an oncofetal protein and has been demonstrated to be associated with aggressive tumor behavior. The aim of this study was to establish the expression pattern and diagnostic value of IMP3 in Barrett esophagus, dysplasia, and carcinoma. A total of 217 cases (resection, n=56; biopsy, n=161) with 302 lesions (invasive esophageal adenocarcinoma, n=147; metastatic esophageal adenocarcinoma, n=14; high-grade dysplasia of the esophagus, n=52; low-grade dysplasia of the esophagus gland, n=21; and Barrett esophagus, n=68) were examined by immunohistochemistry for IMP3 expression. IMP3 showed strong cytoplasmic granular staining in 138 of 147 (94%) of invasive esophageal adenocarcinomas, 13 of 14 (93%) of metastatic esophageal adenocarcinomas, and 49 of 52 (94%) of high-grade dysplasias. In contrast, 3/21 (14%) of low-grade dysplasia and 5/68 (7%) Barrett esophagus were positive for IMP3. Expression of IMP3 was not found in adjacent benign squamous and glandular mucosa. Our findings indicate that IMP3 is a highly sensitive and specific biomarker for the diagnosis of invasive esophageal adenocarcinoma and high-grade dysplasia. The data also suggest that IMP3, an oncofetal protein, may play an important role in malignant transformation in esophageal adenocarcinoma.