PURPOSE: To evaluate the role of enhanced MR imaging in monitoring tumor response to preoperativechemotherapy for osteosarcomas. MATERIALS AND METHODS: Forty-seven patients (30 males and 17 females, witha mean age 17 years ; range 8 -44 years) with osteosarcomas were included in this study. We obtained spin echoT1-, T2-, and enhanced T1-weighted images before and after pre-operative chemotherapy and in all patientscorrelated changes in MR parameters with histopathologic response. We also obtained 19 specimen MR images,correlating these with histopathologic results in order to estimate tissue specific signals. Patients with morethan 10% viable tumor in the resected specimen were considered poor respon-ders(n=26), while those with 10% orless viable tumor were considered good respon-ders(n=21). RESULTS: Four distinct patterns of signal intensitycorresponded, respectively to dead bone and dense fibrosis (low on T1- and T2-weighted images), viable tumor cells(in-termediate on T1- and high on T2-weighted images), necrosis (low on T1- and high on T 2 - weighted images),and hemorrhage (high on T1- and T2-weighted images), but a wide range of overlap was noted. In all four groups,viable tumor cells remained. Increased tumor vo l u m e, stable or increased edema and enhancement were goodpre-dictors of poor response (predictive values of 83%, 77%, and 89%, respectively). Decreased enhancement was theonly reliable predictor of good response (predictive value, 73%). Changes in tumor margin, homogeneity, signalintensity, and joint effu-sion did not correlate with histopathologic response. CONCLUSION: Signal intensities donot reflect histologic nature. Enhanced MR imaging is a useful predictor of tumor response to preoperativechemotherapy.