Pigmented villonodular synovitis (PVS) is characterized by diffuse or localized proliferation of the synovial membrane in synovial joints, tendon sheaths or bursae. The etiology and pathogenesis are unknown. The condition is most frequently localized to the flexor tendon sheaths. Intraarticular PVS is most frequent in the knee joint. Swelling of the joint and slight pain develop. The localized intraarticular form may result in locking of the joint. The joint fluid is frequently haemorrhagic. The laboratory tests reveal normal findings. X-rays reveal erosion of the bone and cyst formation. Histological examination of the affected tissue is necessary to establish the diagnosis. Treatment consists of surgical excision. In the diffuse intraarticular form, this must take the form of total synovectomy. In cases with recurrence or pronounced destruction of bone, joint replacement may be indicated.