Pigmented villonodular synovitis (PVS) is a rare disease of the synovial layer in joints, seen particularly in children. Early diagnosis allows treatment with resection of the affected synovial tissue, avoiding secondary osteoarthritic lesions of the normally unaffected joint structures. In this article, symptoms, clinical findings, and the diagnosis procedure are reported in a rare case of diffuse PVS in an 11-year-old girl. Diagnostic procedures are of differing value, and it seems that the most important factor is suspicion of this disease even in children. Since preoperative verification of the histological diagnosis is impossible, early invasive investigation such as arthroscopy are recommended to confirm the histological character of the lesion. Arthrotomy or arthroscopy followed by total synovectomy is recommended as the surgical treatment of first choice. Our patient was treated by total synovectomy via an arthrotomy of the affected knee joint after histological diagnosis has been confirmed by arthroscopic excision and histological analysis. Follow-up examinations 4 and 6 months postoperatively revealed no signs of recurrence.