An 8-month-old girl underwent surgical resection of a cervical mass with histologic diagnosis of a primitive myxoid mesenchymal tumor of infancy (PMMTI). More than 5 years after the initial surgical intervention, the tumor recurred locally, with numerous distant metastases. The histologic morphology of this tumor was compatible with a diagnosis of an undifferentiated high-grade sarcoma. PMMTI is a recently described poorly differentiated fibroblastic soft-tissue tumor of infancy, of at least borderline biological behavior, characterized by local recurrence and a potential to metastasize. We present here the first case of a transformation of a PMMTI into an undifferentiated high-grade sarcoma.