A study was undertaken to determine the incidence of subclinical infection in Peace Corps volunteers (PCVs) maintained on a routine prophylactic immune globulin (IG) regimen. Serum markers to hepatitis A virus (HAV) were detected in 172 of 1,508 (11.4%) single serum samples from a cross-sectional sample of PCVs. Of those tested prior to service, 15/298 (5%) were seropositive for HAV. Those tested at 1–12 months showed 48/417 (11.3%) seropositivity, while 72/529 (13.6%) of those with >12 months of service had antibody to HAV. Only 3/80 (3.8%) of those seropositive PCVs who received their IG injections at least every 6 months were documented to have shown clinical signs of HAV infection during service. This compares to 6/30 (20%) of those who had received irregular prophylaxis (P = 0.01, Fisher's exact test, 2-tailed). Significant numbers of PCVs are subclinically infected with HAV overseas and increasingly develop active immunity with greater length of service.