Introduction - Between December 2005 and June 2006, a major dengue outbreak occurred in French Guiana. A dengue seroprevalence survey was conducted to estimate the proportion of the population that had been exposed to the virus either recently or in the past. Methods - A cross-sectional survey was conducted over four months among all pregnant women giving birth in the department who had been living there for at least six months. The study included a blood sample test for anti-flavivirus IgM and IgG antibodies as well as a questionnaire. Results - Among the 689 women included, testing for anti-flavivirus IgM showed that 1.9% [95% CI: 0.9%–2.9%] had developed a recent infection, likely associated with the dengue virus. Testing for anti-flavivirus IgG showed that 92.0% [95% CI: 90.0%–94.0%] had been exposed to a flavivirus in the natural environment at least once. Discussion - These results are difficult to interpret since the population is theoretically vaccinated against yellow fever and cross-reactive serological responses exist. Given the low circulation of other flaviviruses in the natural environment, these results suggest that approximately 92.0% of the pregnant women in French Guiana have been exposed to a dengue virus. (R.A.)