We analysed data from a multicentre interferon (IFN) treatment trial to evaluate symptoms in patients with chronic hepatitis C and to identify factors that might predict development of debilitating IFN side-effects. Two hundred and twenty-two patients (120 US, 102 French) received 3 or 5 million units (MU) of IFN-α three times weekly (t.i.w.) for 3 months. Those who had detectable hepatitis C virus (HCV) RNA, as detected by the branched DNA signal amplification (bDNA) assay, at 3 months were intensified to daily therapy, while patients who were bDNA negative continued t.i.w. dosing for the subsequent 3 months of treatment. Symptoms were assessed at baseline, and adverse effects were evaluated at 6 months of therapy. Prior to treatment, the most common symptom that interfered with daily functioning was fatigue, occurring in 25% of patients. The frequency of debilitating fatigue, myalgia, arthralgia, headache, the presence of dry eyes and dry mouth, and use of antidepressant medication increased significantly from baseline to 6 months of IFN therapy (all P < 0.01). In multivariate analysis, the development of a debilitating side-effect at 6 months of treatment was associated with the presence of that symptom prior to therapy in all cases. Symptoms and adverse effects varied by gender and country. Compared with patients maintained on t.i.w. dosing, those who were dose intensified to daily IFN reported more debilitating fatigue, malaise, myalgia, arthralgia, fever, nausea, and headache, and the presence of dry mouth (all P < 0.05). In conclusion, patient characteristics, including pretreatment symptoms, gender and nationality, as well as daily IFN dosing are associated with the development of debilitating adverse effects on IFN therapy.