Chronic hepatitis B infection with the hepatitis B e antigen (HBeAg)-negative variant is associated with a severe clinical course and a low response rate to interferon (IFN). In an attempt to improve the chances of sustained response to interferon we designed a pilot study, using titres of IgM antibodies to hepatitis B core antigen (HBcAb IgM) to guide treatment initiation. Eighteen adults who were HBeAg-negative with biopsy-proven chronic active hepatitis (seven with cirrhosis) entered the study. They were followed-up bimonthly with routine liver function tests, and HBcAb IgM titres were also determined. Treatment (lymphoblastoid IFN 5 million units (MU) m(-2) three times weekly for 6 months) was started when the HBcAb IgM titre was increasing. Fifteen (83.3%) patients had normal alanine aminotransferase (ALT) levels and undetectable HBV DNA at the end of treatment. HBcAb IgM decreased in all responders. We observed a relapse in four patients (three with cirrhosis), in the first year after treatment, with an increase in ALT, HBV DNA and titre of HBcAb IgM. Eleven patients (61.1%) had a sustained response and eight of these 11 patients were followed-up for more than 18 months; two responders cleared hepatitis B surface antigen (HBsAg). Hence, the rate of sustained response to IFN in HBeAb-positive patients with chronic hepatitis is improved if treatment is started when HBcAb IgM levels are increasing.