Thirty patients with definite rheumatoid arthritis and hydropsy in a knee joint were followed for 42 months in a prospective study. The knee joints were initially aspirated and 15 synovial fluid variables investigated. The patients were split into two groups, those with and those without progress of radiologically detected destruction in the knee joints during the follow-up. Of the synovial fluid variables at the start synovial fluid proteins (P = 0.002) and acid phosphatase (P = 0.03) differed statistically significantly between the groups, both being higher in patients with worsening of knee joint. The results suggest that high synovial fluid proteins and acid phosphatase are predictors of poor prognosis in a joint affected by rheumatoid arthritis.