[A swelling in the knee cavity, not caused by a Baker's cyst but by a nerve sheath tumour].

In a woman, aged 30 years, who presented with a right popliteal mass, a Baker's cyst (popliteal cyst) was diagnosed. Five years later she developed symptoms that were attributed to compression of the peroneal nerve by the mass. Because of the troublesome nature of these symptoms, it was decided to excise the mass. Immediately after the operation, a complete loss offunction ofthe common peroneal nerve was apparent. Histopathologic examination revealed an ancient schwannoma (nerve sheath tumour). The most important reason for not having diagnosed the schwannoma was that it was not considered in the differential diagnosis of the popliteal mass. Although Baker's cysts are the most common popliteal masses, nerve sheath tumours should also be considered in the differential diagnosis.