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Serum levels and 24-hour urinary excretion of β2-Microglobulin (β2-M) was investigated in a group of 101 patients with rheumatoid arthritis (RA), without any other signs of renal disease in past or present, andin a comparable control group. Elevated 24-hour urinary β2-M excretion, due to renal proximal tubules dysfunction, was observed in 19% of the patients and not in the control group. There was a significant correlation with clinical signs of extra-articular RA. It is postulated that the observed increase may be an early symptom of renal involvement in RA. Elevated serum β2 levels, corrected for glomerular filtration rate, were observed in 44% of the RA patients and only in 3% of the control group and correlated with clinical signs of a more severe RA, as well as with increased 24-hour urinary β2-M excretion. [ABSTRACT FROM AUTHOR]