The aim of this study was to assess early (15 min) and late (2 h) thallium-201 (Tl-201) uptake in children with osteosarcoma and to compare these findings with magnetic resonance imaging (MRI) and technetium-99m methylenediphosphonate (Tc-99m MDP), with emphasis on evaluating tumor viability before and after chemotherapy. Fifteen patients with biopsy-proven osteosarcoma received standard preoperative chemotheapy with a combination, of cisplatin, Adriamycin, and high-dose methotrexate. Their ages ranged between 7 and 18 years (median 14.5 years). All patients had Tl-201, Tc-99m MDP, and MRI studies. Thallium scintigraphy was performed at 15 min and 2 h after IV injection of 92 MBq of thallium. Thallium uptake ratio was calculated by dividing the count density of the lesion (L) by that of the controlateral normal (N) area. The percent reduction of Tl-201 uptake ratio (alteration ratio) was calculated by [100 x (prechemotheropy L/N - postchemotherapy L/N)/prechemotherapy L/N]. Pathologic changes were graded on the basis of % tumor necrosis as defined histologically. Scintigraphic comparisons demonstrated a high-degree of correlation with late Tl-201 alteration ratio and poor correlation with both early Tl-201 and Tc-99m MDP alteration ratios. Late Tl-201 images were superior to early Tl-201, Tc-99m MDP, and MRI in predicting tumor response to chemotherapy as determined by % tumor necrosis (p < .01). The authors found that late Tl-201 was an accurate test for evaluating the response to specific therapeutic regimens and it can be useful planning surgery or choosing alternative chemotherapeutic regimens.