Synovial sarcoma accounts for 8-10% of all of the soft tissue sarcomas; it’s characterized by high risk of local relapse, even after surgical complete excision, deceiving onset ed slowly growth. Generally arising in the contest of joint or from immediately surrounding anatomical sites, first of all affecting inferior limbs (2/3), a sarcoma of the knee, elective anatomical site, is described, accidentally diagnosed after traumatic event. Exhibiting a very poor 5 year survey, (55%), related to dimension, distal or proximal arising, necrosis rate and grading, it’s the most frequently soft tissue malignancy misdiagnosed with benign neoplasms, such as Baker cyst or villonodular pigmented synovitis, considering its deceiving macroscopic and chronological features; the differential diagnosis seems to be very hard, relying on histhological biopsy. Though the normal conventional x-ray finding, clinical examination and anamnesis have suggested in our patient the ultrasonographic investigation in urgency, which revealed the suspected nature of the lesion, which must be distinguished by hematoma, much more frequent thraumatic pathology, inducing severe prognosis in false negative cases.