Magnetic resonance imaging findings were compared with histopathologic specimens of ossified or calcified ligaments, including the surrounding tissue, to evaluate the usefulness of magnetic resonance imaging as a means to predict the progression of ossification. In addition, factors influencing signal intensity were evaluated by histochemical and immunohistochemical analyses. The area of low signal intensity corresponded to the hyperplastic ligament around the ossification, and to the transitional area between the ligament and the ossification. A mineralization front and chondrocyte proliferation with strong metachromasia were recognized in these areas. The presence of a low-intensity signal area suggested the progression of ossification. The isointensity signal corresponded to proliferation of small vessels in the hyperplastic ligament, presumably representing the initiation of ossification. Histochemically, the metachromasia was intense at the transitional area between the ligament and the ossification, and the main constituent in these areas was chondroitin sulfate. Immunohistochemically, S-100 protein, transforming growth factor-β1, and Type II collagen showed an intense immunoreactivity in the chondrocytes, indicating increased production of the extracellular matrix. We postulate that the activation of chondrocytes and the alteration of the extracellular matrix may have affected the signal intensity.