Hepatitis B virus (HBV) is an important cause of morbidity in military forces. HBV threatens a safe blood supply for mass casualties, contributes to rising health care costs and present an occupational risk to health care workers. The suceptibility of soldiers reflects that of other residents of their country of origin. In intermdediate and high-prevalence regions, minfections usually occur before the age of military service. In low-prevalence regions, HBV infections usually occur in young adults and follow exposure to an HBV carrier through blood or sexual contact. Control of HBV infections must rely on education regarding risk factors, screening of blood donors and immunization. Military populations provide unique oportunities for studying the epidemiology of hepatitis B and carrying out mass vaccination programmes. © 1990.