Aplastic anemia is a rare, life-threatening disease of unknown etiology, with unusually high prevalence in Thailand. It is sometimes associated with non-A, non-B hepatitis (NANBH). The hepatitis C virus (HCV), one of the causes of NANBH, is similar to flaviviridiae, a family of viruses many of whose members cause acute bone marrow suppression. To test the hypothesis that HCV viremia is associated with aplastic anemia among patients in Thailand, we compared 53 untransfused hospitalized aplastic anemia patients and 39 untransfused controls hospitalized for other conditions. We used the polymerase chain reaction to identify HCV viremia in three (5.7%) untransfused patients and two (5.1%) untransfused controls (P = 1.0, by Fisher's two-tailed exact test). Although our data do not exclude the possibility that a small subset of aplastic anemia cases are precipitated by HCV, we conclude that HCV viremia is not generally associated with aplastic anemia in Thailand. Our results also imply that the prevalence of HCV viremia may be unexpectedly high among untransfused persons in Thailand, a hypothesis that should be tested in other populations.