The magnetic resonance (MR) changes after one cycle of chemotherapy were compared with the prechemotherapy studies in 39 patients with osteosarcoma, in order to identify MR changes which can be used to assess early response to chemotherapy. Measurements of total tumour volume, signal intensity, and tumour enhancement of the intra- and extraosseous tumour component were performed. Change in the amount of oedema was subjectively assessed. Changes observed were correlated with histopathological response. Changes in tumour volume and of the signal intensity of the extraosseous tumour component on T2-weighting were the only two parameters which correlated significantly (P < 0.05) with histopathological response. Increase of tumour volume is the most significant parameter and indicates poor response (sensitivity 89%, specificity 73%). Only one good responder showed increase of tumour volume. Decreased or stable tumour volume was observed in both good and poor responders. Increase of signal intensity was found exclusively in five poor responders (sensitivity 100%, specificity 23%). Decreased or stable signal intensity was observed in both good and poor responders. Changes in the amount of oedema and contrast enhancement could not predict response at an early stage. We conclude that increase of tumour volume and increase of T2 signal intensity of extraosseous tumour can be predictive for poor response. MR criteria are not helpful in the early identification of good responders.