Rheumatoid pseudocysts of which pathogenesis are not well known, are commonly observed in the major joints of the body, especially knees in rheumatoid patients. This report investigated the pseudocysts in the knee of a rheumatoid patient radiologically and immunohistochemically. A sixty-three-year-old woman with rheumatoid arthritis who had pseudocysts in the femoral lateral condyle and tibial plateau was hospitalized to have the lesions surgically removed. The MRI findings showed a connection between the pseudocysts and the joint cavity. A window of 1 x 1 cm2 was made at the tibial anterior cortex and the contents of the pseudocyst were resected and hydroxyapatite granules were inserted into the lesion after the removal of the pseudocyst tissue. In the histological findings, the specimen from the tibial pseudocyst revealed fibrous connective tissue with a few inflammatory cells, while the synovial specimen revealed the fibrous tissue with intense inflammatory cells infiltration forming lymphoid follicules. In immunohistochemical findings, both specimens showed positive with anti-HLA class I, anti-HLA DR, anti-CD44 and anti-HSP70 antibodies. The pathological findings of the pseudocystic lesion were similar to those of synovia, and it was considered that the synovia had invaded into the bone and formed the pseudocystic lesion. The MRI findings also support this hypothesis. The pseudocystic lesion was surrounded by bone and it was isolated from cytokain-rich joint effusion. This isolation from rheumatic joint effusion may explain the weaker inflammation of the pseudocystic lesion than that of synovia.