The authors describe two patients with knee symptoms. In both cases the correct diagnosis was established late. One patient had a large cyst of the lateral meniscus, while the other had a synovial sarcoma. Aetiology, symptomatology, methods of investigation, therapy and prognosis are discussed, as well as the differential diagnosis. It is stressed, that a meniscal cyst is not adequately treated until the entire meniscus is removed. The puzzling signs and symptoms of a synovial sarcoma of the knee are mentioned. It is obligatory to perform a biopsy in all patients with an unfamiliar swelling of the knee, in order to rule out a malignancy, especially the treacherous synovial sarcoma.