The prevalence and risk factors of hypertriglyceridemia during the administration of interferon (IFN) were examined in 78 patients with chronic hepatitis C who were treated with 6 MU of IFN-β once or 3 MU of IFN-β twice a day for 6 weeks. Hypertriglyceridemia (serum triglyceride (TG) above 150mg/dl) was found before the start of IFN treatment in 9% of the patients. During the administration of IFN, elevation of serum TG above 150mg/dl was found in 82% of patients. In addition, serum TG level exceeded 500mg/dl at least once during the administration of IFN in 13% of patients. On stepwise multiple regression analysis, three risk factors, high serum TG value before the administration of IFN, high ALT value before the administration of IFN, and divided administration of IFN-β twice daily were found to be associated with hypertriglyceridemia during IFN administration.