Pigmented villonodular synovitis (PVNS) is a disease that involves the lining of joints, bursae and tendon sheaths. The incidence is low and estimated to be 1.8 patients per million population. The cause of PVNS is unclear and discussed to be either inflammatory or neoplastic. PVNS has been described in 2 forms different for prognosis and treatment (nodular and diffuse). The articular form almost appears in the knee joint as we describe below in a 14 year-old patient. PVNS was first defined in 1941 by Jaffe e.a. [7]. Because of the uncommon occurrence of the disease it is difficult to amass patient series to allow confirmed statements on therapy and outcome. Larger patient series raise out of long periods of time. Differences in outcome and recurrence rates exist for the nodular and diffuse form. Clinical findings are moderate pain and swelling of joints due to effusion and synovial proliferation. Magnetic resonance imaging shows typical findings. Surgical procedures are recommended as open or arthroscopic synovectomy for the diffuse form of PVNS, local excision for the nodular form and arthrodesis or prosthetic replacement for joint destruction.