The clinical, pathologic, and radiologic features of pigmented villonodular synovitis of large synovial joints are presented. The typical plain-film presentation of this unusual entity is a noncalcified capsular soft-tissue mass of the knee, without bony abnormalities. When pigmented villonodular synovitis occurs in other joints, it is often accompanied by cystic bone erosions. This important radiographic finding might mislead one to an improper diagnosis of neoplasia or infection. This review reveals insufficient emphasis in the radiologic literature on the high incidence of bone lesions in joints affected by pigmented villonodular synovitis. A mechanism for the development of bone cysts is proposed. Clinical, pathologic, radiologic, and therapeutic considerations are discussed.