Hepatitis C virus (HCV) can infect peripheral blood mononuclear cells (PBMC) of patients with chronic HCV infection. No data are available on PBMC testing for HCV RNA in acute hepatitis C. This study investigated the presence of HCV RNA in PBMC of patients with acute posttransfusion hepatitis C, compared with those with chronic HCV infection. Nested polymerase chain reaction (PCR) was applied to detect HCV RNA in 111 and 48 paired samples of serum and PBMC of 11 patients with acute posttransfusion hepatitis C and 48 patients with chronic HCV infection, respectively. In patients with acute posttransfusion hepatitis C, HCV RNA was detected in 17 of 29 (59%) and 67 of 82 (82%) serum samples collected during the incubation period and acute phase, respectively. Meanwhile, of the 48 patients with chronic HCV infection, 41 had serum HCV RNA (85%). HCV RNA was not detected in PBMC samples from incubation period or from acute-phase hepatitis, although it was detected in 12 of the 48 PBMC samples of chronically infected patients (25%) P < .005). Of the 12 PBMC specimens positive for positive-stranded HCV RNA, 6 were also positive for negative-stranded HCV RNA. Among patients with chronic HCV infection, HCV infection of PBMC was not related to age, sex, blood transfusion, serum alanine transaminase (ALT) levels, or serum virus titers. In conclusion, HCV infection of PBMC rarely exists in patients with acute hepatitis C. As HCV infection persists, the incidence of HCV infection of PBMC becomes higher.