Objective. To study the prevalence of hepatitis delta and hepatitis C viruses (HDV and HCV) in patients infected with hepatitis B virus (HBV) with and without risk behaviors for the infection by these viruses and by the human immunodeficiency virus (HIV). Materials and Methods. Clinical and serological study (January 1990-December 1992) at Medicine Service at Galdacano Hospital of 109 patients with positive hepatitis B surface antigen and without clinical or serological evidence of acute hepatitis, 60 without risk behaviors for the infection, 45 parenteral drug abusers (PDA) and 4 homosexual males. Results. Chronic HDV infection was observed in 60% of PDA and more frequently among HIV-positive patients (p<0.05). In six of these patients the serological determination of HDV-Ag was positive. Chronic HDV and HCV infection rates in patients without risk behaviors were 1.6% and 16%, respectively. Eighty-eight percent of PDA had anti-HCV; no differences were observed between HIV-positive and HIV-negative patients (p=0.64). Conclusions. HDV was particularly associated with parenteral drug abuse, with a similar prevalence to other surveys conducted in Spain. The higher frequency of HDV and HDV-Ag among HIV-positive patients could indicate a greater susceptibility to HDV in clinical conditions associated with immunosupression. HCV infection occurs at early stages and is common among PDA infected with HBV, with no apparent association with patient's age, years of drug abuse or HIV infection.