Primary synovial osteochondromatosis is a rare disorder and its diagnosis remains a challenge to the physician. We present the case of 36 year old patient with right knee monoarthritis in which ultrasound findings, corroborated with clinical and histopathological evidence confirmed the diagnosis of unossified primary synovial osteochondromatosis. The arthroscopy with synovectomy found multiple intra-articular loose bodies occupying the entire joint. The ultrasonographic findings were extensively evaluated.