This 9-year-old girl presented with a painful mass in the posterolateral aspect of her right knee. Excisional biopsy of the mass revealed a juxta-articular myxoma. The differential diagnosis for such an intra- or periarticular mass should include meniscal cyst, parameniscal cyst, ganglion cyst, juxta-articular myxoma, focal nodular PVNS, and desmoid tumor. Current imaging techniques do not allow differentiation between these benign lesions and low grade myxoid-containing sarcoma.