The seroprevalence of hepatitis A virus (HAV) antibodies is low in young adults in Korea. From May to July 2005, 17 cases of HAV were reported from healthcare workers (HCWs) in a hospital intensive care unit (ICU). We looked for the presence of anti-HAV IgM from all patients in the medical-surgical ICU with elevated aspartate aminotransferase (AST) and alanine aminotransferase (ALT) and screened AST and ALT levels in all HCWs who came into contact with two suspected index cases. Once the outbreak was confirmed, the molecular subtypes of HAV from the blood of HCWs were determined. Index cases and a transmission route were identified, and intervention strategies applied to control the outbreak. The 17 HCW cases included 13 nurses and four doctors aged 22-32 years, who each suffered acute HAV infection during the study period. The possible transmission of HAV was via the faecal-oral route from bedridden patients with diarrhoea. All HCWs were positive for anti-HAV IgM and eight were positive for HAV RNA. Analysis of the VP1-2A region of each isolate showed genotype IA in five strains and co-circulation of genotypes IA and IB in the others. This HAV outbreak highlights the importance of standard infection control precautions within a hospital. Molecular study of patients' blood would be useful for clarifying the epidemiology of a suspicious HAV outbreak in a hospital.