KAI-1 expression in pediatric high-grade osteosarcoma. Recent evidence implicates cell surface proteins of the tetraspanin superfamily in the process of metastasis whereas the downregulation of KAI-1, a member of the tetraspanin family, is associated with an aggressive clinical phenotype in several types of human cancers. To determine if expression of KAI-1-1 is associated with any known prognostic marker or clinical outcome in high-grade osteosarcoma, we examined 91 nondecalcified archival samples from 47 patients for the expression of KAI-1. Archival, paraffin-embedded, and decalcified pathologic samples were examined by immunohistochemistry and results were correlated to clinical outcomes and known prognostic markers. There were 46 samples from diagnostic biopsies (1 diagnostic sample was not available), 32 tumor resection samples, and 13 metastasis samples. Thirty-three percent (n=30) of the samples expressed KAI-1 (16 biopsies, 9 resections, and 5 metastasis). KAI-1 expression was not significantly related to known prognostic markers or to either tumor necrosis after neoadjuvant therapy or the incidence of metastasis at diagnosis. KAI-1 expression was not significantly different between paired diagnostic tumor samples and either resection or metastasis tumor samples. Twenty-five patients remain alive at a median follow-up of 95 months. The overall and progression-free survival percentages at 5 years were 62% and 47% for KAI-1-positive patients and 49% and 38% for KAI-1-negative patients, respectively. This difference was not statistically significant. We conclude that KAI-1 is expressed in a proportion of high-grade osteosarcoma but is not of clinical significance and cannot be used to stratify treatment groups for these patients.