Laboratory procedures were compared with brain biopsy findings in 113 biopsy-proven patients with herpes simplex virus (HSV) encephalitis and 93 biopsy-negative individuals. Examinations of brain tissue by histopathology, immunofluorescence, and electron microscopy demonstrated evidence of HSV infection in 56070, 70070, and 45070 of proven cases and apparently false-positive results in 14070,9070, and 2070 ofthose biopsynegative. Serologic assessments revealed that HSV encephalitis occurred as both a primary (30070) and recurrent (70070) infection. Among patients with HSV encephalitis, 28070 failed to seroconvert or seroboost within one month of the onset of disease. Titers of passive hemagglutinating and IgG immunofluorescent antibodies increased fourfold in the cerebrospinal fluid in 74070 and 94070, respectively, of patients with proven disease. Similar percentages of patients had antibody ratios in serum and cerebrospinal fluid of ˜20 over the same interval. These data indicate the need for development of noninvasive diagnostic procedures. © 1982 by the University of Chicago.