Two patients with histologically verified acute hepatitis but without any serological evidence of hepatitis A or hepatitis B infection are described. In both cases the acute attack of hepatitis type 'non-A, non-B' progressed histologically and clinically to chronic active hepatitis within a two-year period. One of the patients died from liver insufficiency a year later, while the other is still alive after eight years of follow-up. The two cases illustrate that a progression of acute hepatitis 'non-A, non-B' to chronic liver disease may occur just as has been reported for hepatitis B infection.