Spontaneously growing testicular seminoma in the aged rat was imaged by one of the most sensitive imaging modalities, namely, phase-contrast X-ray computed tomography (CT) with crystal X-ray interferometry. Phase-contrast X-ray CT clearly depicted the detailed inner structures of the tumor and provided 20× magnified images compared to light-microscopic images. Phase-contrast X-ray CT images are generated based on density variations in the object, whereas pathological images are based on differentiation of cellular structures, such as the cellular nuclei and cytoplasm. The mechanism of image generation differs between the two techniques: phase-contrast X-ray CT detects even minute differences in the density among pathological structures, depending, for example, on the number and sizes of the nuclei, variations of the cytoplasmic components, and presence/absence of fibrous septa, cystic changes, and hemorrhage. Thus, phase-contrast X-ray CT with a spatial resolution of 26 µm might allow prediction of the morphological characteristics of a tumor even before histopathological processing.