Few data are available to evaluate the performance of existing assays for antibody to the hepatitis E virus (anti-HEV). A panel of 164 randomized and coded sera was tested for anti-HEV by 12 different assays. The panel included a dilution series of an early convalescent human serum, known-positive sera (undiluted human sera obtained 2 months to 13 years after acute hepatitis E, and postinoculation chimpanzee sera), known-negative sera (preinoculation chimpanzee sera; sera from chimpanzees with hepatitis A virus, hepatitis B virus, or hepatitis C virus infection; and normal human sera), and sera obtained from previously tested U.S. blood donors without a history of hepatitis. Six tests detected anti-HEV in > or =90% of undiluted known-positive sera. The sensitivity of all of the assays with known-positive sera ranged from 17% to 100%, and the limit of detection by endpoint dilution ranged from 1:5 to 1:160. Ten tests were nonreactive for all of the 22 known-negative sera, one test was reactive for one serum, and one test was reactive for 5 sera. In pairwise comparisons of different tests in blood donor sera, the overall concordance ranged from 49% to 94% (median, 69%) and the concordance among reactive sera ranged from 0% to 89% (median, 32%). Several of these tests performed well in detecting anti-HEV in known positive sera. However, highly discrepant results among U.S. blood donor sera indicate that anti-HEV seroprevalence data in non-HEV-endemic countries may be unreliable and should be interpreted with caution.