['Acute exacerbation of chronic hepatitis B has to be distinguished from acute hepatitis, because treatment strategies differ between them.', 'Mutations in the core promoter and precore region of hepatitis B virus (HBV) were determined in 36 patients with acute exacerbation of chronic hepatitis B, in whom alanine aminotransferase (ALT) increased above 500 IU/L, as well as the 36 patients with acute hepatitis.', 'Mutations in the core promoter (A1762T/G1764A) and precore region (G1896A) were more frequent in patients with acute exacerbation of chronic hepatitis than acute hepatitis (81% vs 19%; P < 0.0001 and 58% vs 6%; P < 0.0001, respectively). Of the 19 patients with mutations in both the core promoter and precore region, 17 (89%) had acute exacerbation of chronic hepatitis. In contrast, among the 32 patients with the wild-type for both the core promoter and precore region, 29 (89%) developed acute hepatitis. By multivariate analysis, the double mutation in the core promoter was predictive of acute exacerbation in chronic hepatitis with the highest odds ratio at 26.4.'] Does detection of HBV core promoter and precore mutations help distinguish flares of chronic hepatitis from acute hepatitis B?