The response of osteosarcomas to preoperative chemotherapy as measured by quantitative 99mTc-diphosphonate scintigraphy was compared with the amount of tumor cell destruction found histologically in the resected specimens. 3-phase bone scintigraphy, with determination of tumor/non-tumor (T/NT) ratios, allowed accurate prediction of tumor response in 28 of 30 tumors (accuracy = 93%) after completion and in 10 of 12 osteosarcomas (83%) at the half-way stage of preoperative chemotherapy, when only those tumors were evaluated which showed convergent changes of T/NT ratios in the perfusion and the mineral phase. At the half-way stage, however, evaluation of chemotherapy effects was complicated in 12 of 23 osteosarcomas by diverging T/NT ratios. In contrast, quantification and mapping of the tumor plasma volume and 99mTc-diphosphonate clearance on parametric images predicted correctly the histological tumor response to chemotherapy in 10 of 11 tumors (91%) after completion and in all 11 cases (100%) at the half-way stage.