Infection with hepatitis B virus (HBV) is much more common in tropical than in temperate countries. Visitors to the tropics are thus at risk from HBV, though the degree of risk, and the routes of infection involved are uncertain. We report serological markers of HBV in two groups of 2nd World War soldiers, who served in the Thai/Burma jungles. The groups comprised 100 ex-prisoners of the Japanese (POW), and 100 Burma Campaign Veterans (BCV). Surface antigen to HBV (HbsAg) was positive in 0% of POW and 2% of BCV (P = not significant). Surface antibody (anti-HBs) and core antibody (anti-HBc) were both positive in 40% POW and 13% BCV (P less than 0.001). Quoted UK prevalence rates for these markers are 0.1% for HBsAg, 1.5% for anti-HBs and 0.7% for anti-HBc. Both groups thus show very high rates of past HBV infection. For the POW there were many possible reasons, including contaminated surgical instruments and needles, blood transfusions, and multiple beatings with common weapons. None of these factors operated significantly for BCV. Malarial transmission was, however, intense in both groups, though more so in POW. The data thus again raise the possibility of horizontal transmission of HBV by biting insects in tropical countries.