Root resorption is a very important finding in the radiographic diagnosis of the oral and maxillofacial region. However, there are few reports about root resorption related to malignant tumors. It is a commonly accepted theory that a malignant tumor does not cause root resorption because its invasion is too rapid. However, we have encountered many unexpected cases of root resorption associated with malignant tumors. In this study, we investigated the frequency and shape of root resorption in malignant tumors and considered the relation between tumor extension and root resorption. The subjects were 72 patients with pathologically diagnosed malignant tumors. Radiography showed that the roots of their teeth were in contact or involved with the tumors. We observed the radiographs and classified the findings into 2 types as follows: (1) resorption of alveolar bone (site of tumor invasion): 1. alveolar-margin type 2. root-apex type. (2) resorption of tooth root: 1. spike type 2. knife-edge type. The root resorption rate for all cases was 43% (31/72). (There were 19 squamous cell carcinomas, 2 adenoid cystic carcinomas, 3 malignant lymphomas, 2 metastatic tumors, 1 poorly differentiated carcinoma, 3 undifferentiated carcinomas and 1 fibrosarcoma.) Root resorption rate of alveolar-margin-type tumors was 37% (19/51), and that of the root-apex-type tumors was 57% (12/21). All alveolar-margin-type tumors showed spike-type root resorption, and all root-apex-type tumors were of the knife-edge type except 1 case. Our study suggested that root resorption associated with malignant tumors is not rare and that the shape of root resorption may be related to the site of tumor invasion. In the case of root resorption with resorption of alveolar bone, it is important to take malignant tumors into consideration as well as other diseases.