Abstract : Pigmented villonodular synovitis (PVNS) of the knee is a benign but locally aggressive disease of synovial proliferation that occurs in localized nodular and diffuse villous growth patterns. Although inflammatory and neoplastic causes have been hypothesized, etiology remains unknown. PVNS usually presents as unilateral knee pain and swelling and mimics other knee ailments. Radiographs are often unremarkable, whereas magnetic resonance imaging may show characteristic intra-articular masses with signal dropout on T2-weighted sequences. The condition is surgically treated with open or arthroscopic total or partial synovectomy. High recurrence rates are associated with all treatments of diffuse PVNS. Intra-articular radioisotope injection and external beam radiation may be beneficial adjuvant therapy for extensive diffuse and recurrent PVNS of the knee. We report a rare case of pigmented villonodular synovitis in a 56-year-old woman, who presented with a painful mass in the posterior part of the left knee.