Telomerase expression predicts unfavorable outcome in osteosarcoma. Osteosarcoma is distinct from most cancers in that the majority of osteosarcomas lack telomerase expression and use the alternative lengthening of telomeres (ALT) mechanism to maintain telomeres. Laboratory studies suggest that compared with ALT, telomerase expression is associated with increased tumor aggressiveness. We evaluated the clinical significance of telomerase expression in human osteosarcoma.Fifty-six osteosarcomas from 51 patients treated at St Jude Children's Research Hospital between 1982 and 2003 were evaluated for telomerase enzyme activity, mRNA expression of the catalytic component of telomerase (TERT), and presence of the ALT pathway.Outcome analysis was based on TERT mRNA expression in the primary tumor samples from 44 patients. Fourteen primary tumors expressed TERT mRNA (32%; eight TERT only, six TERT and ALT) and 30 did not express TERT mRNA (68%; 29 ALT, one no ALT). Progression-free survival (PFS) was inferior in the TERT-positive group compared with the TERT-negative group (3-year estimates, 21.4% +/- 9.5% v 63.7% +/- 11.1%; P =.014). Likewise, overall survival was inferior in the TERT-positive group compared with the TERT-negative group (3-year estimates, 42.9% +/- 12.2% v 70.0% +/- 9.9%; P =.031). Among 31 patients with nonmetastatic disease at diagnosis, PFS was lower in the TERT-positive group compared with the TERT-negative group (3-year estimates, 33.3% +/- 13.6% v 72.0% +/- 11.5%; P =.092).Telomerase expression in primary tumor samples is associated with decreased PFS and OS in patients with osteosarcoma. Additional studies are warranted to better define the clinical utility of this molecular marker.