Unexplained chronic hepatitis (CH) in the adult liver allograft recipient is not uncommon, but its natural history and clinical significance is unknown. A retrospective study was undertaken of adult liver allograft recipients to determine the frequency and natural history of unexplained CH. We evaluated only those patients who had undergone >or=2 liver biopsies after 6 months and were grafted for indications where recurrent disease could be confidently excluded (alcoholic liver disease [ALD] in those who remained abstinent and fulminant hepatic failure [FHF] from drug reactions). Of 288 patients who were transplanted for ALD or FHF, 30 fulfilled the above criteria. CH was first diagnosed at a median of 15.25 months after transplantation. A total of 24 patients showed mild necroinflammatory changes, and 12 had mild or moderate fibrosis. Liver tests did not reflect the presence or degree of inflammation or fibrosis. After a median of 4 years, necroinflammatory scores were increased in 5; new or progressive fibrosis was noted in 13%; 3 had developed cirrhosis; and 5 developed clinical evidence of portal hypertension. Progressive fibrosis was associated with a high titer of anti-nuclear antibodies (>1:1600) and a portal tract plasma cell infiltrate. There was a trend for correlation between necroinflammatory activity and fibrosis stage, but this did not reach statistical significance (P = 0.06). Serum alkaline phosphatase (P = 0.012) and female gender of the donor (P = 0.033) were associated with progressive fibrosis. Unexplained CH is not uncommon in the liver allograft and may progress to established cirrhosis in a subgroup of patients transplanted for ALD or FHF. Standard liver tests do not reflect the extent of these changes, so protocol liver biopsies may be required to detect these changes. We recommend careful history and follow-up in these patients.