Acute hepatitis A (AH-A) and acute hepatitis B (AH-B) were found more severe in males and females, respectively, while impacts from underlying liver disease on severity were not excluded in the AH-A study. The precise gender-specific impact on the severity of AH-A was investigated and compared with that of AH-B. A case-control study of overt AH-A (n = 118) and AH-B (n = 118) patients without any underlying liver disease was conducted. Overt hepatitis was defined as serum bilirubin ≥ 2mg/dL and alanine transaminase (ALT) ≥ 10 × upper limit of normal. Of the AH-A patients, age (95% confidence interval of odds ratio 1.051-1.147) and ALT (1.001-1.002) were associated with hepatic decompensation. Indifferent rates of hepatic decompensation, hepatic failure, and mortality were found between male and female patients. Compared with the AH-B patients, AH-Apatients showed lower bilirubin levels (p < 0.001), hepatic decompensation (p = 0.004), and mortality rates (p = 0.013). Among patients < 40years, the AH-A patients had higher hepatic decompensation rates than AH-B in the male subgroup (15% vs. 2.8%, p = 0.045), while the situation is reverse in the female subgroup (7.7% vs. 48.1%, p = 0.001). Overt AH-A was less severe than overt AH-B and, unlike AH-B, had no difference in severity between males and females. Among subgroups < 40years, AH-A was more severe than AH-B in males, but the situation was reverse in females in terms of hepatic decompensation rates.