The risks of developing hepatitis A and B virus infections were studied in 233 French volunteers working for 18 to 35 months in the field in West and Central Africa. During their stay in Africa, the seroconversion rates to antibody to hepatitis A virus of the immunoglobulin G (IgG) class and hepatitis B marker(s) were 48.1% and 10.5%, respectively; 21.5% of the volunteers developed jaundice, and 78.0% of the jaundice cases were associated with hepatitis A seroconversion, 14.0% with hepatitis B seroconversion, 4.0% with seroconversion to both, and 4.0% without seroconversion. Male health workers were at lower risk than other male occupations for hepatitis A infection. Female health workers were at higher risk than other female occupations for hepatitis A infection. Health workers were at a lower risk of hepatitis B infection, but the significance was borderline. Populations such as these volunteers should receive immunoprophylaxis against hepatitis A and B infections.