Pigmented villonodular synovitis is a benign disease of the synovial membrane of joints, tendon sheaths, or bursae, which nevertheless can cause marked local destruction. Its diagnosis is often delayed because complaints and symptoms are nonspecific. Familiarity with the disease may ensure an earlier diagnosis and consequently early onset of therapy, which may prevent serious damage. This paper describes 18 patients suffering from localized or diffuse pigmented villonodular synovitis. Findings possibly suggestive of pigmented villonodular synovitis include hemarthrosis, soft tissue swelling, radiological evidence of cyst formation at a distance from the weight-bearing area of a joint, an increased triglyceride concentration, and a positive bone scan. A normal appearance on arthroscopy does not rule out the disease. Therapeutic results are better in the localized than in the diffuse form of the disease.