We describe the case of a malignant arthritis of the knee. This is the first description with an oesophageal adenocarcinoma being the primary site of the malignancy. Diagnosis of this rare condition can be made by cytological examination of joint fluid or by synovial biopsy. X-ray of the joint can, as in our case, be negative. There is no standard therapy and the prognosis seems to be poor. Although rare, malignant synovitis represents an important differential diagnosis in patients with an unexplained monoarthritis, as it can be the first sign of malignancy or the only site of metastic disease.