[en] Usefulness of MRI in monitoring preoperative chemotherapy effect in patients with osteosarcoma was evaluated. Six patients with conventional type osteosarcoma were studied. MRI was performed before and after courses of chemotherapy, and MRI-pathologic correlation was made after operation. Images were obtained with SE method except for one case, in which dynamic MRI was performed with FLASH method following a bolus contrast injection. Disappearance of the peritumoral reactive zone, clearification of the tumor margin, appearance of the marginal low signal intensity band, and decreased size of the extraosseous mass are indicative of a good response to the chemotherapy on MRI. However, an increased signal intensity of the extraosseous mass and an enhancement effect with contrast injection on SE images are not always indicative of a poor response. Most of these high intensity and/or enhanced areas consist of postnecrotic granulation tissue and loose fibrosis with proliferation of dilated small vessels. An early phase of dynamic MRI after a bolus contrast injection could delineate tumor vascularity in the largest tumor cut plane. Dynamic MRI seems to be a promising diagnostic imaging in evaluating preoperative chemotherapy effect. (author)