Pediatric undifferentiated soft tissue sarcomas (USTSs) are tumors composed of primitive mesenchymal cells. As such, they form an attractive model for studying the early events in sarcoma development. In an effort to better understand the critical molecular aberrations leading to sarcoma development, gene expression array analysis and post-array validation techniques were applied to several USTSs; the results were consistent with upregulation of the excitatory components of the insulin-like growth factor (IGF) pathway. Particularly high expression of the insulin-like growth factor 2 (IGF2) ligand was seen and confirmed by real-time reverse transcriptase-polymerase chain reaction. Immunohistochemistry performed using antibodies against IGF2 revealed overexpression of the IGF2 protein in 19 of 21 (90%) USTSs and revealed 2 distinct staining patterns, 1 in which there was diffuse cytoplasmic staining (16/19) and 1 in which there was punctate perinuclear positivity (3/19). Using ultrastructural immunogold localization of IGF2, it was determined that IGF2 was primarily localized to Golgi-derived vesicles and multivesicular bodies in tumor cells showing the punctate pattern, and to both the cytoplasm and plasma membrane of tumor cells showing the diffuse pattern. The results suggest that upregulation of the IGF pathway in pediatric USTSs is a critical early event in the development of sarcomas. Furthermore, findings from the immunocytochemical and immunogold analyses confirm the presence of 2 different cytoplasmic trafficking patterns and storage motifs of IGF2 within this type of tumor. Given that in one subcellular pattern the IGF2 protein does not appear to reach the membrane, these findings may have functional significance.