The safety and efficacy of granulocyte-macrophage colony-stimulating factor (GM-CSF) as adjuvant therapy for interferon alpha (IFN-alpha) treatment has been evaluated in 20 non-cirrhotic patients with chronic hepatitis C virus (HCV) infection, Adjuvant therapy with GM-CSF plus IFN-alpha was associated with less myelosuppression than with IFN-alpha alone (P < .01), although the rate of local adverse reactions increased, GM-CSF adjuvant therapy led to a 50% biochemical response (transaminase values within the normal range at therapy end) and to reductions in HCV RNA concentrations (median HCV RNA reduction of 99%, range 8-100%), which were similarly observed in single IFN-alpha recipients (median HCV RNA reduction of 91%, range 38-100%). However, HCV RNA became undetectable in three biochemical responders to the GM-CSF adjuvant therapy but in only one biochemical nonresponder to IFN-alpha alone, The use of GM-CSF as adjuvant therapy is safe and, although it has not improved the biochemical response, it might potentiate the virologic response to IFN-alpha treatment alone. (C) 1996 Academic Press Limited