Various kinds of hematological abnormalities have been known to occur in liver diseases. To understand the hematological changes in acute viral hepatitis, 324 adults with acute viral hepatitis were studied. Of them, 3 were acute hepatitis A, 91 acute hepatitis B, 99 acute non-A, non-B hepatitis (NANB) and 181 acute hepatitis on chronic hepatitis B (AH on CH-B). There were 233 males and 91 females; age ranged from 16 to 74 years (mean age 39 years.) The results showed the incidences of thrombocytopenia (platelet less than 120,000/cmm), anemia (Hb less than 12 g% in male and less than 10% in female patients), leukocytosis (WBC greater than 10,000/cmm) and leukopenia (WBC less than 4,000/cmm) were 19.3%, 12.6%, 10.8% and 7.4%, respectively. Patients with AH on CH-B had significantly higher incidence of anemia and thrombocytopenia than those with acute B hepatitis; other than this, there was no significant difference. Patients with anemia, thrombocytopenia or leukocytosis had significantly higher mean levels of serum bilirubin and higher proportions of prolonged prothrombin time, suggesting that these hematological abnormalities were closely related to the severity of hepatocellular damage. In addition, there were 3 cases (0.9%) complicated with aplastic anemia. Two were NANB hepatitis and the other was AH on CH-B which was seronegative for anti-delta, possibly suggesting NANB virus superinfection. Of these 3 cases, 2 died of complications related to aplastic anemia and 1 survived with normal hematological findings 148 days later.