Forty four rheumatoid arthritis (RA) patients with hydrops of the knee joint had partial synovectomy of the knee joint performed under arthrosopy and subsequent irrigation of the joint with 3,000 ml physiological salile solution. Seventeen out of these 44 cases demonstrated recurring hydrops at one year after the procedures (the recurrence group). The remaider (n = 27) was regarded as the non-recurrence group. The histological features consistent with RA (proliferation of surface cells, palisade fasion, multi-nucleated giant cells, lymphocytes, plasma cells and granulation tissue; and the occurrence of fibrinoid necrosis and ferritin depotits) were graded depending on the severity of each feature, according to Koizumi's criteria. The rate of heavy lymphocyte and plasma cell infiltration in the non-recurrence group was significantly higher than that in the recurrence group (P = 0.00 and 0.001, respectively). Therefore, we concluded that the prognosis for one-day arthroscopic partial synovectomy on rheumatoid knee joints is related to the pathological features of the synovial membrane.