Synovial fluid (SF) of patients with rheumatoid arthritis (RA) has been noted to contain cholesterol crystals and increased amounts of cholesterol compared with normal SF. SF, plasma apolipoproteins (apos) A-I and B, and cholesterol in 12 untreated classic RA patients (inflammatory arthritis) and eight untreated degenerative joint disease ([DJD] noninflammatory arthritis) patients were analyzed. Results showed that mean apo A-I, apo B, and cholesterol levels of RA SF were significantly higher than those of DJD SF (apo A-I, P = .004; apo B, P = .0008; cholesterol, P = .0004). Regression analyses of plasma and SF apo A-I and apo B (r = .72, P = .008 and r = .63, P = .02, respectively) suggested an increased permeability for these lipoprotein constituents across RA synovial membrane that was not observed in DJD synovial membrane. These data suggest that RA synovium but not DJD synovium is more permeable to major apoproteins of low- and high-density lipoproteins (LDL and HDL). These apolipoproteins have been shown to influence the immune response and may therefore be involved in the pathogenesis of RA.