Hepatitis B virus (HBV) is known to show significant genetic diversity. There are eight HBV genotypes (A-H) characterized by distinct geographical distribution. Mutations in the HBV genome, in particular precore (PC) and basal core promoter (BCP) mutations, may be important factors in the pathogenesis of disease. In this study genetic heterogeneity and phylogenetic analysis of HBV isolates from 32 naïve patients with acute HBV infection was investigated. Eleven patients presented with severe infection, while the remaining 21 had self-limiting illness. Only four isolates from patients with severe HBV infection harbored the G1896A stop codon mutation. One isolate (Irish-13), collected from a patient with acute asymptomatic infection, had a G1896A mutation and a 243 bp deletion of the polymerase gene. A triple mutation, T1753C/A1762T/G1764A was identified in only one isolate (Irish-3) associated with severe infection. The latter also had a mutation, A2339G, in the core gene, not previously reported in severe acute infection caused by genotype D. Variations within the S gene were identified in 6 isolates, including Gly145Ala, associated with vaccine immune escape, Asp144Glu, Ser143Leu and Phe134Leu, each associated with failure to detect HBsAg. Phylogenetic analysis was determined using amplicons of the S gene (678 bp) and distal-X/PC region (672 bp). Genotype A was the most common (75%), followed by genotype D (15.6%), and equal proportions of C, E, F, and H. A novel recombinant of genotypes D and E was identified in an isolate originating from West Africa. Genetic heterogeneity of HBV isolates of HBV isolates from patients with acute infection needs further study of its significance.