The activity of rheumatoid arthritis was assessed in 96 unselected out-patients by performing a number of clinical measurements and comparing them to ESR, plasma and serum viscosity. It was found that ESR correlated with clinical parameters better than did both plasma and serum viscosity. These tests were repeated 3 months later in 87 of the same patients and again change in ESR correlated with change in disease activity better than did change in plasma viscosity. Plasma viscosity was a consistently more reliable measure of disease activity than was serum viscosity.