Lipoprotein metabolism was investigated in 69 patients with untreated active rheumatoid arthritis (n = 48) and in seronegative spondylarthropathies (n = 21). The patients had high inflammatory activity as measured by erythrocyte sedimentation rate and C-reactive protein (CRP). Serum cholesterol and cholesterol levels in the very-low-density lipoprotein (VLDL), low-density lipoprotein, and high-density lipoprotein fractions were reduced by 20% to 30% compared with healthy controls; and triglyceride levels in VLDL and high-density lipoprotein were reduced by 10% to 30%. There were significant correlations between the inflammatory activity and certain lipoprotein lipids, ie, between CRP and VLDL triglycerides, VLDL cholesterol, and serum triglycerides. The fractional elimination rate (K2) measured by an intravenous fat tolerance test was 30% higher in the patients than in the controls despite reduced tissue lipoprotein lipase activities. There was correlation between CRP and the K2 value. These findings suggest that it is the degree of inflammatory activity that governs the altered lipoprotein metabolism in untreated active chronic inflammatory arthritides. The relationships between CRP and VLDL and between CRP and K2 suggest that the VLDL particles may be altered by inflammatory process, and that the increased elimination may take place through the "scavenger pathway."