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Background: The incidence of acute hepatitis A infection in Japan peaked 10 years ago and has been decreasing since then. However, an increase in severe cases of the disease has been documented recently. We experienced an outbreak in 1998–1999, and compared the clinical features of the disease in 1998–1999 (recent outbreak) and in 1987–1988 (past outbreak) in our prefecture (Gunma). Methods: Forty patients with acute hepatitis A were admitted to nine Gunma hospitals from October 1998 to September 1999. Their clinical features were compared with those of 100 patients with acute hepatitis A admitted to the same hospitals in 1987–1988. Results: Both outbreaks occurred mostly during the winter-spring season. Secondary familial infection was significantly decreased in the recent outbreak. Patients in the recent outbreak were 7 years older than those in the past outbreak. Laboratory findings, such as serum aspartatate aminotransferase (AST) and alanine aminotransferase (ALT) levels and prothrombin time, were worse in the recent than in the past outbreak. Severe-type hepatitis and fulminant hepatitis occurred in 5 patients (12.5%) in the recent outbreak but in only 2 patients (2.0%) in the past outbreak. Conclusions: Clinical data and manifestations were more severe in the recent outbreak than in the past outbreak of acute hepatitis A. It is important to be aware of hepatitis A virus infection and to take into account the available vaccination against hepatitis A virus in Japan. [ABSTRACT FROM AUTHOR]