Ipilimumab is a monoclonal antibody that inhibits the CTLA4 receptor on cytotoxic T lymphocytes, resulting in immune-mediated tumor cell death. Ipilimumab is most often used in the treatment of metastatic melanoma, and rarely liver toxicity necessitating cessation of treatment occurs. The aim of this study was to characterize the histologic features and clinical course of ipilimumab-associated hepatitis. Eleven patients with clinical suspicion of ipilimumab-induced hepatitis, due to the development of abnormal liver function tests (LFTs) while receiving treatment, and who underwent liver biopsy, were identified over a 6-year period. Ten patients were male and 1 female (median age 58 y), and all received 1 to 4 doses of ipilimumab. None had known preexisting liver disease. Two patients were obese, and another had a history of alcohol abuse. Viral and autoimmune serologies were negative in all patients except 1 who had a mildly elevated ANA titer. Nine biopsies showed active hepatitis with 2 distinct histologic patterns: panlobular hepatitis in 6 cases and zone 3 hepatitis in 3. The inflammatory infiltrate was similar in composition in both patterns, composed predominantly of CD8 T lymphocytes, admixed histiocytes, scattered plasma cells, and eosinophils. Prominent histiocytic sinusoidal infiltrates were present in 7 cases and frequently formed loose histiocytic aggregates. Central vein endothelialitis was present in 8 cases. Patients in this group tended to have markedly elevated ALT, AST, and total bilirubin. Two cases did not fit into the above 2 histologic groups: 1 showed portal inflammation with cholangitis, and the other showed morphologic features indistinguishable from nonalcoholic steatohepatitis. Discontinuation of ipilimumab and administration of immunosuppressives resulted in resolution or marked improvement of LFTs in all patients within 3 months of presentation. Ipilimumab may potentially unmask previously subclinical liver disease, for example, fatty liver disease, and the diagnosis of ipilimumab-induced liver injury may only be recognized with certainty after cessation of the drug leads to normalization of LFTs. Overall, ipilimumab-associated hepatitis most often presents with a panlobular active hepatitis that resembles autoimmune hepatitis. Prominent sinusoidal histiocytic infiltrates and central vein damage with endothelialitis may be helpful histologic clues to the diagnosis of ipilimumab-associated hepatitis.