Heat shock protein 72 expression in osteosarcomas correlates with good response to neoadjuvant chemotherapy. Although the therapeutic outcome of osteosarcoma patients has improved dramatically within the last 20 years because of combined neoadjuvant chemotherapy and surgery, the problem of drug resistance remains. Thus far, markers that can predict the response to chemotherapy at the time of biopsy are not available. Heat shock proteins (hsp) 60, 72, and 73 have been shown to play a role in tumor immunity, and our study investigated their expression in human osteosarcomas and nonmalignant bone tumors before neoadjuvant chemotherapy. Immunohistochemical evaluations of hsp expression was performed on paraffin-embedded sections of 45 patients (17 female, 28 male, aged 6.5 to 62 years; mean, 19.4 years) with high-grade osteosarcoma at the time of biopsy, before preoperative chemotherapy. These results were correlated to histological response to chemotherapy, tumor size, age, alkaline phosphatase serum levels, and duration of symptoms. Thirty-four patients (15 male, 19 female, mean age 27 years) with osteoblastoma, osteoid-osteoma, or fibrous dysplasia served as nonmalignant controls. Hsp60 was uniformly found in the cytoplasm of both benign and malignant bone tumors. Nuclear hsp73 expression quantitatively increased in osteosarcoma cells. Hsp72 was significantly overexpressed in osteosarcomas (17 of 45, 38%) compared with nonmalignant bone tumors (1 of 34, 2.9%; P < .001). Hsp72-positive osteosarcomas responded better to neoadjuvant chemotherapy than hsp72-negative cases (P < .001), co-express hsp60, and correlate with higher tumor size (P < .005) and location in the distal femur. No differences were observed relative to age, gender, duration of symptoms, alkaline phosphatase levels, or hsp73 expression between hsp72-positive and hsp72-negative tumors. Hsp72 expression seemed to be a predictive immunohistochemical marker for osteosarcoma, because it is the first marker to prospectively correlate to response to neoadjuvant chemotherapy. It therefore, may be of importance in preoperative therapy regimens for nonresponding high-risk patients.