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Purpose: To clarify the CT characteristics and histopathological findings of pancreatic ductal adenocarcinomas that are not detected in early-phase contrast-enhanced CT images., Materials and Methods: The CT findings of eight patients with histopathologically proven pancreatic carcinomas that were not detected in early-phase images following the rapid injection of contrast material were reviewed. The examinations consisted of pre-contrast-enhanced CT and multi-phase contrast-enhanced CT, with thin-section scanning in each patient. The CT findings were compared with those of the resected specimens., Results: In all cases but one, the lesion was in the pancreatic head. In seven cases, the tumor did not appear as a focal area of hypoattenuation compared with surrounding pancreatic parenchyma in early-phase images, and in the remaining case, small areas of poor enhancement were observed. In late-phase images, hyperattenuated and isoattenuated areas were seen in six and two cases, respectively. In all but one case, the lesion was less than 40 mm in size. All lesions were composed of acinar tissues and tumor cells, and contained small amounts of mucin, necrotic tissue, and pus. The fibrous tissues were not abundant and were interlobular and/or relatively loose., Conclusion: Pancreatic ductal adenocarcinomas showing isoattenuation in early-phase images tend to be relatively mild lesions with scirrhous and/or desmoplastic changes.