Increasing use of liver transplantation and new treatment regimens necessitate an accurate estimate of prognosis in primary biliary cirrhosis. To test the usefulness of the Mayo model for this purpose, the R value of the model was calculated for a group of 28 patients after each patient encounter and plotted against time. The data were best described by two linear regressions. For the period 10-12 years before death, the average increase in R value was 0.23 annually [R = 7.1 - 0.23 × time (in years)]. In the last 2 years of life, the average increase in R value was 1.4. This period could be fit by the expression R = 8.2 - 1.4 × time (in years). The increase in R value represents the natural progression of primary biliary cirrhosis and can be used for evaluating treatment of patients.