The prevalence of chronic hepatitis and its different aetiologies was studied in all patients above 10 years of age seen in the specialised outpatient clinic of our hospital over a 3 year period. Defining chronic hepatitis as a persistent elevation of the transaminases, 988 patients (62% of all the patients observed) were classified as follows: viral aetiology in 82%, metabolic in 2%, biliary in 2%, alcoholic in 11%, autoimmune in 1.5% and idiopathic in 2%. Among the viral group, hepatitis B virus infection was predominant (65%), followed by hepatitis C virus (26%) and delta hepatitis (8%). While the hepatitis C and delta patients presented high transaminases, in the HBsAg carriers this occurred in 94% and 20% of the HBeAg and anti-HBe-positive patients, respectively. Thirty per cent of the patients with chronic hepatitis B, 35% of those with chronic hepatitis C and 18% with delta hepatitis were selected for alpha interferon therapy. This demonstrated that in a significant proportion of patients with chronic viral hepatitis, therapy with interferon is not indicated. Corroborating other studies, with even stronger data, our study shows that viral aetiology is the most frequent type of chronic hepatitis.