This study assessed the use of ribavirin monotherapy to enhance sustained virologic response in hepatitis C virus (HCV)-infected patients who achieved virologic response to interferon (IFN)-ribavirin combination therapy. Patients who had chronic HCV infection and prior relapse were retreated with IFN-ribavirin for 6 months. Patients with an end-of-treatment virologic response were assigned randomly to either stop use of both IFN and ribavirin or to continue use of ribavirin as monotherapy for an additional 6 months. HCV RNA became undetectable during treatment in 46 patients, who then entered the randomized trial. Sustained virologic response was observed in 13 of 26 patients who continued ribavirin monotherapy and in 15 of 20 patients who stopped use of both IFN and ribavirin (P, not significant). Sustained virologic response was significantly more common in patients with HCV genotype non-1 (75% vs. 56%) and in patients with a virus titer < 2 x 10(6) copies/mL (93% vs. 43%). The results indicate that continuing ribavirin monotherapy after achieving a virologic response does not improve sustained virologic response.