Modafinil may be a potentially effective treatment for primary biliary cirrhosis (PBC)-related fatigue. About 42 patients were given a 3-day trial of 100-200 mg modafinil. Response was defined as increased energy, decreased somnolence and sleep requirements, and improved daily function. Patients with positive responses were continued indefinitely on the medication. During the initial trial period, 31 (73%) patients had complete response and continued to take the medication. Eleven (26%) had no response. In long-term follow-up (average 17.7 months), 25 (81%) patients continued to take 100-200 mg modafinil daily. Some required an increased dosage and some took the medication as needed. Four (12%) patients stopped the medication because of side-effects or reduced efficacy; one patient (3%) stopped due to medication cost and one (3%) due to resolution of fatigue. Side-effects included insomnia, nausea, nervousness, and headaches. Modafinil appears to be a safe, effective treatment for PBC-related fatigue.