We followed up 55 patients with proven primary biliary cirrhosis for several years or until death. A graph of the level of serum bilirubin versus time that was constructed for each patient shows an initial period of variable length in which the serum bilirubin level remained constant. This was followed by a period of rapid rise in serum bilirubin which culminated in the patient's death. Whenever two successive serum bilirubin values taken six months apart exceeded 34 mumol/l (2.0 mg/dl) the patient had entered a late phase of disease and lived an average of 49 months. Ninety-five per cent confidence limits on survival time were 32-74 months. If two successive six month bilirubin values exceeded 102 mumol/l (6.0 mg/dl), calculated survival time was 25 months, and if two successive six month bilirubin values exceeded 170 mumol/l (10.0 mg/dl), survival time was 17 months. Fifteen of the 41 living patients had two consecutive serum bilirubin levels greater than 34 mumol/l (2.0 mg/dl). However, the slope of the rising bilirubin in the living patients is only 35 mumol/l/yr (1.5 mg/dl/yr) compared with 42 mumol/l/yr (2.5 mg/dl/yr) in the dead patients. This means that patients with this disease not may be living considerably longer.