The prevalence of hepatitis D virus infection was studied in 954 sera samples collected in Spain between 1974 and 1986 from 838 patients in a variety of epidemiological categories with HBsAg-positive liver disease, and from 116 haemophiliacs and drug addicts with hepatitis B antibodies. Hepatitis D markers were detected in 64% of drug addicts and 33% of HBsAg-positive haemophiliacs but were absent in asymptomatic chronic carriers, homosexual males, mentally retarded patients and haemodialysis patients. Hepatitis D markers were found in 6% of patients with acute hepatitis, in 5.6% of those with chronic hepatitis B from general population, and in 65% of those with acute hepatitis B and 67% of those with chronic hepatitis B among drug addicts. During the 12-year study, no changes were observed in the frequency of Delta infection. The prevalence was less than 10% in chronic HBsAg-positive hepatitis. These findings indicate that in Spain Delta infection is restricted mainly to HBsAg-positive drug addicts and multitransfused haemophiliacs, while in the general population only 6.7% of those with chronic hepatitis B show D superinfection. Anti-HD sera detection is correlated with more severe forms of chronic hepatitis such as chronic active hepatitis and cirrhosis.