Graft dysfunction associated with autoimmune phenomena has been recently described in liver transplant recipients without previous autoimmune disease. However, the natural history, diagnostic criteria, and definitive therapeutic approach of de novo autoimmune hepatitis (de novo AIH) are poorly understood. We report 12 cases of de novo AIH 27.9 +/- 24.5 months after liver transplantation: the outcome of 7 patients treated with steroids is compared with a group of 5 nontreated patients. Nontreated patients lost the graft after 5.8 +/- 2.6 months from de novo AIH onset. All treated patients were alive after 48.4 +/- 14 (29-65) months from de novo AIH onset, and none of them lost the graft. However, 5 patients relapsed in relation to steroid tapering. All patients presented an atypical antiliver/kidney cytosolic autoantibody, associated to classical autoantibodies in 10 cases. Histological study showed several degrees of lobular necrosis and inflammatory infiltrate. HLA antigen frequencies and matching were compared with 2 control groups (16 orthotopic liver transplantation [LTX] patients without de novo AIH and 929 healthy blood donors); de novo AIH patients showed a higher prevalence of HLA-DR3 (54.5% vs. 25.9%, P =.04) than healthy controls, which was not observed in LTX patients without de novo AIH. In conclusion, this new disease should be included in the differential diagnosis of unexplained graft dysfunction. In addition, treatment with steroids results in a dramatically improved outcome. However, maintenance therapy is usually required.