One-hundred-and-two out of 788 consecutive patients (12.9%) hospitalized for an attack of acute hepatitis fulfilled criteria for non-A, non-B (NANB) hepatitis, and were followed prospectively for 8 to 70 months. Forty-one out of 97 patients showed a monophasic pattern of alanine aminotransferase (ALT), and all recovered completely. In contrast, 22 of 56 (39.2%) subjects with a polyphasic pattern of ALT exhibited persistent hypertransaminasemia for more than 14-18 months, and all of them developed chronic hepatitis. Thus, a polyphasic pattern of ALT seems to characterize one of two forms of NANB hepatitis, more frequently associated with parenteral exposure, absence of jaundice and a high tendency for the development of chronic hepatitis. This suggests distinct immunopathogenetic mechanisms from two unrelated NANB agents. Benign, non-progressive chronic persistent hepatitis is the prevalent form of chronic sequela of NANB hepatitis observed in our patients. Although many of these patients show a tendency to spontaneous remission of the disease, there is the possibility in some of a deterioration of inflammatory activity of the liver that has need of immunosuppressive therapy.