A prospective study of the clinical pattern and course of primary biliary cirrhosis based on the data of 236 patients (211 females, 25 males) in an international randomized trial has been performed mainly using life-table technique. The median follow-up period has been 18 mo. After the entry into the study, at which time the median duration of symptoms was 2.1 yr, the estimated 5-yr increase in the cumulative percentage of patients was for pruritus from 75% to 95%, jaundice 59% to 82%, hepatomegaly 54% to 86%, pigmentation 54% to 85%, xanthomas 27% to 50%, GI bleeding 17% to 46%, ascites 7% to 49%, and vertebral collapse 3% to 20%. The frequency of cirrhosis increased from 30% to 82% in 4 yr. The mitochondrial antibody titer showed a high rate of progression with time. In analysis of subgroups, age, histologic stage, and particularly the serum bilirubin level were important prognostic factors. Sex, duration of symptoms, and character of first symptom or sign had no independent prognostic influence. The most important indication of seriously progressive course was rapid development of severe cholestasis independent of the histologic stage.