The prevalence of HBV, HDV and HIV infection was studied in 288 imprisoned intravenous drug addicts (IVDA) and 329 controls. Commercially available radioimmunoassays for the detection of HBV and HDV serologic markers and enzyme-immunoassays for IgM anti-HBc and anti-HIV were used. Anti-HIV positive results were confirmed by Western Blot. The prevalence of HBV serologic markers among IVDA (77.1%) was found considerably higher than among controls (22.5%). An increased prevalence of HBsAg carriers (6.9%) and anti-HBc alone positives (9.7%) was also found. IVDA carriers were more frequently HBeAg positive (25.0%) and HDV serologic markers were detected in 35.0% (7/20) of them. Anti-HIV were detected in 6 (2.1%) IVDA but in none control. These data show the widespread HBV and HDV infections among Greek IVDA and suggest the need for continuation of the initiated hepatitis B vaccination program. Anti-HIV prevalence is yet low. However there is urgent need for an extensive campaign to limit the further spread of the HIV among this high risk group.