In a study of the presence of delta-Ag in liver biopsies of HBsAg positive patients with chronic liver diseases living in the Naples area, delta-Ag has been found only in patients with HBsAg positive chronic active hepatitis and in none of HBsAg positive patients with other liver diseases including those with chronic persistent hepatitis. The data obtained support the hypothesis that the simultaneous infection with HBV and delta-agent, and superinfection with delta-agent in HBsAg positive patients predispose the outcome of chronic hepatitis.