The demographice of primary biliary cirrhosis in Ontario, Canada, are described. Two hundred and twenty-five primary biliary cirrhosis patients were identified by 86 of 602 gastmenterologiete (or interniete) practicing in Ontario acute care hoepitals that have 150 or more beds. Two hundred and six patients were verified as being antimitochondrial antibodypositive, resulting in an incidence of 3.26 per million per year and a prevalence of 22.39 per million. Questionnaire data were obtained on 88.6% of t h e e patients. Twenty-nine percent of the patients were found to be asymptomatic. Geographical clustering and racial prediepoeition were not seen. No increase in b r e d cancer prevalence was noted. By the time the diagnosis of primary biliary cirrhosis was established, the patients had consulted a median number of 3.6 physicians. Fatigue was reported aa the most disabling symptom. The diagnosis of primary biliary cirrhosis in patients referred from across the province of Ontario was independently confinned by us, using standard criteria (antimitochondrial antibody testing and liver biopsy), and wae found to be reliable. (HEPATOLOGY 1990,12:98-105.) Epidemiological studies conducted in the United Kingdom, United States, Europe and Japan (1-14) indicate that the prevalence of primary biliary cirrhosis (PBC) varies between countries and between districts within the same country (6). Furthermore, recent reports show that the prevalence of PBC may be increasing (9, 10). Although the term PBC was first coined in the Canadian literature by Dauphinee and Sinclair in 1949 (151, no Canadian study has addressed the demography of PBC in Canada. We, therefore, elected to study the demographics of PBC in Ontario,