We report a case of an 8-year-old boy who initially presented with a painful subungual mass that was diagnosed as a "glomus tumor" largely based on the clinical presenta tion. Local excision was performed, and the patient remained tumor free for 17 years, after which he experienced three local recurrences and massive pleural metastases within the next 2 years. The tumor was poorly differentiated with small round cells and increased mitotic activity on light microscopy. The precise nature of the tumor was difficult to characterize and was unraveled primarily by immunoperoxidase studies, particularly the diffuse strong positivity of HBA-71 (MIC 2) antibody, also called pe ripheral neuroectodermal tumor-Ewing's sarcoma antigen, which is reported to be fairly specific for these tumors, and the expression of neural markers Leu-7 (HNK-1 ) and neuron-specific enolase. In addition, ultrastructural examination revealed imma ture cell junctions, cell processes, and abundant pools of intracytoplasmic glycogen. Metastatic pleural tumor recurred very rapidly after excision, but showed a dramatic, although short-lasting, response to chemotherapy with cisplatin and etoposide. The thumb is an uncommon location for peripheral neuroectodermal tumor-Ewing's sar coma, and a 17-year, disease-free interval is most unusual for a malignant tumor of this nature. We discuss this case with respect to its unique clinical behavior and in the context of the differential diagnosis of small, round cell neoplasms. Int J Surg Pathol 2(2):147-156, 1994