Iodide clearance was measured in the chronic knee effusions of 11 patients with rheumatoid arthritis and 9 patients with osteoarthritis. The mean (+/- SE) iodide clearance of 1.92 +/- 0.30 ml/minute in rheumatoid arthritis effusions did not differ significantly from the 2.19 +/- 0.52 ml/minute found in osteoarthritis effusions. Clearance values in rheumatoid arthritis patients ranged widely (0.79-3.22 ml/minute). Iodide clearance in these patients correlated directly with synovial fluid (SF) pH (r = 0.731, P = 0.005), SF glucose:serum glucose ratio (r = 0.746, P = 0.004), and SF temperature (r = 0.878, P = 0.001), and directly with SF lactate (r = -0.782, P = 0.002) and percentage of SF neutrophils (r = -0.581, P = 0.03). These relationships support the hypothesis that rheumatoid synovitis is often marked by tissue hypoperfusion, and that the "sickest" rheumatoid joints (as defined by physiologic indices) are the most ischemic.