The clinical and serological course of 35 instances of hepatitis B (22 hepatitis B alone and 13 with delta co-infection) was comparatively and prospectively studied for a mean follow-up period of 7.19 +/- 2.69 months. There were no significant differences between the clinical presentation and course between both types of patients. There were not, either, outstanding differences in the mean bilirubin and transaminase levels between both types of hepatitis, although the patients with delta co-infection initially showed a more pronounced cholestatic pattern than did patients with hepatitis B alone (p less than 0.05). In hepatitis B with delta co-infection, the serological markers of activity and/or replication of hepatitis B virus (HBV) showed an earlier serum clearance than in patients with hepatitis B alone, basically in relation with the "e" antigen of HBV. Serum measurement of hepatitis delta virus (HDV) antigen by means of ELISA was shown to be a high sensitivity test (69.23%) for the diagnosis of acute HDV hepatitis, which was carried out in an early stage since the development of symptoms.