To determine the predictive factors for rheumatoid arthritis (RA), 79 patients (11 men, 68 women; average age at onset of symptoms 37.1 years) with fixed joint effusion of one knee joint, of minimum 6 months' duration, were divided into three groups: group I, 11 patients (14%) who progressed to RA; group II, 8 patients (10%) with the correct diagnosis, except that RA became apparent during the subsequent follow-up; group III, 60 patients (76%) whose joint effusion resolved. In group I, the degree of joint effusion and the serological values of interleukin (IL)-1β, IgG-RF, and rheumatoid factor (RF) tended to be higher than those in the other groups at the time of our initial examination. The synovial fluid concentrations of IL-1β and IgG-RF in group I were significantly higher than those in the other groups. Magnetic resonance imaging (MRI)-determined stage and histological assessment of synovial inflammation also tended to be higher in group I than in the other groups. This study revealed that it might be possible to predict the outcome of cases of monoarthritis by examining IL-1β and IgG-RF levels in the synovial fluid, in addition to various elevated inflammation signs in the knee joint.