Hepatitis E virus (HEV) is the major etiologic agent of enterically transmitted viral hepatitis in many developing countries. Epidemics are primarily waterborne in areas where water supplies are contaminated with HEV of human origin. There is increasing evidence, however, that HEV is also prevalent in very low numbers in non-endemic countries, including Japan. Although the source of HEV in these sporadic cases is unknown, a recently isolated swine virus is the best candidate for causing a zoonotic form of hepatitis E. The virus is serologically cross-reactive with human HEV and genetically very similar, and the human and swine strains seem to be cross-infective. Very recent evidence has also shown that swine HEV, and possibly a deer strain of HEV, may be related to avian HEV and HEV in other hosts and potential reservoirs. We examined the prevalence of anti-HEV IgM and IgG among patients serologically diagnosed with non-A, non-B, non-C acute hepatitis (n = 126) and compared with a matched control group of 76 individuals. Enzyme-linked immunosorbent assay revealed a significant difference in seroprevalence between the two groups for anti-HEV IgM (5.6% versus 0%), whereas there was no difference for anti-HEV IgG (21.4% versus 26.3%). For confirmed cases of anti-HEV IgM we also detected HEV RNA in sera by means of a sensitive polymerase chain reaction (PCR) assay. This study provides evidence of locally acquired hepatitis E in the Chiba area. Therefore, in cases of unexplained acute hepatitis, the diagnosis of hepatitis E should be considered even in the absence of foreign travel.