The general aim of the study is to identify a safe and effective therapy for fatigue in patients with primary biliary cirrhosis. Specific aims include: 1) to determine the safety profile of modafinil in patients with primary biliary cirrhosis; 2) to evaluate the beneficial effects of modafinil on patients with primary biliary cirrhosis and fatigue as documented by a well-validated questionnaire (Fisk Fatigue Severity Score) to be applied at the beginning and end of the study period; and 3) to compare the performance of three questionnaires (Fisk Fatigue Severity Score, Fatigue Severity Scale and the fatigue domain of the PBC-40) as instruments to determine the benefits of a therapeutic intervention on fatigue by establishing the minimally important clinical difference of these measures of fatigue.