The objective of this open, controlled clinical trial was to assess whether Yellow Fever (YF) vaccine would interfere with one single dose of an inactivated hepatitis A (HAV) vaccine 1440 El.U. when they are administered simultaneously. One hundred and ten healthy adults (24 +/- 4 years old, 65% females) were included and randomized to receive one single dose of HAV vaccine (Group 1) or HAV vaccine plus YF vaccine (Group 2). All subjects received a booster dose of HAV vaccine (month 6). Anti-HAV antibodies were measured (ELISA method) at screening and at months 1, 6 and 7. One month after the first inoculation, the seroconversion rates and geometric mean titers for anti-HAV were 98% and 275 mIU ml-1 in Group 1 and 100% and 239 mIU ml-1 in Group 2. After the booster dose all vaccinees had become seropositive. Administration of YF vaccine did not alter the safety profile of HAV vaccine. These results indicate that simultaneous administration of YF vaccine does not interfere with the immune response to HAV vaccine (1440 El.U.) in young healthy adults.