The value of plasma renin and its inactive precursor, prorenin, were examined as a marker for vasculitis in rheumatoid arthritis (RA). Plasma renin and prorenin rise when the renin-angiotensin system is activated; an isolated increase of prorenin may be a marker for microvascular complications in diabetes mellitus. Renin concentrations in plasma obtained from 34 patients with RA (seven with vasculitis, 27 controls) were measured under standard conditions, before and five days after stopping non-steroidal anti-inflammatory drugs; creatinine clearance was also measured. At first the median renin concentration in the patients with vasculitis was 19 (range 12-63) mU/l (normal less than 61 mU/l) and in the controls 9 (3-43) mU/l. The median prorenin concentration in patients with vasculitis was 233 (144-428) mU/l (normal less than 358 mU/l) and in the controls 144 (25-364) mU/l. Renin and prorenin concentrations increased significantly in both groups after withdrawal of nonsteroidal anti-inflammatory drugs. The creatinine clearance was similar in both groups and did not correlate with renin concentrations. In conclusion, it was found that, unlike patients with diabetes mellitus, patients with RA with vasculitis had slightly raised concentrations of both renin and prorenin. These findings signal activation of the renin-angiotensin system and might indicate early cardiac or renal involvement by vasculitis.