Herpes simplex virus (HSV) hepatitis is rare in adults, usually occurring in immunocompromised individuals and in otherwise healthy women in the third trimester of pregnancy. Three cases of HSV hepatitis occurring in pregnant women were diagnosed at our institution between 1981 and 1990. This diagnosis was not suspected clinically, and in each case was made on the basis of histology, immunoperoxidase studies, and viral cultures of liver tissue. Clinically, the patients had severe anicteric liver failure with markedly elevated serum aspartate aminotransferase and alanine aminotransferase levels; two of the three patients died. None had mucocutaneous lesions at the time of diagnosis. Histologically, two distinct patterns of necrosis and inflammation were seen. Two of the cases had well-demarcated foci of necrosis scattered randomly throughout the lobules with neutrophilic infiltration, giving the impression of abscess formation. Hepatocytes at the periphery of these areas of necrosis had enlarged nuclei with "ground-glass" inclusions; however, no Cowdry type A inclusions were seen. Rare multinucleated cells were present. Immunoperoxidase staining using antibodies to HSV was positive primarily in the hepatocytes with inclusions. The third case had diffuse, almost total hepatic necrosis with no viral inclusions and virtually no inflammatory response. This histologic pattern is similar to that seen in neonates with HSV infection. Immunoperoxidase studies in this case were negative; however, viral cultures were positive. While HSV hepatitis may be suspected or diagnosed on the basis of histology alone, viral cultures are an important adjunct since viral inclusions may be absent. Prompt diagnosis is important since antiviral therapy is now available.