Six patients positive for both human immunodeficiency virus (HIV) and hepatitis B were studied to assess the effect of dideoxyinosine (DDI) on hepatitis B virus (HBV) replication. Two patients died during the follow-up period and four had at least 8 weeks of therapy. One patient demonstrated HBV DNA suppression and became transiently negative. In the remaining five patients, there was no appreciable change in HBV DNA levels during DDI therapy. DDI was well tolerated in all patients, the only significant side effect being diarrhoea. It is concluded that DDI has no notable antiviral effect in patients with chronic HBV infection when coinfected with HIV.