Undocumented migrants face major barriers in accessing prevention and health care. Whereas the association between low socioeconomic status and poor health is well documented only few studies have addressed specific health issues in undocumented migrants. The aim of the present study is to describe sexual and reproductive health behaviors of undocumented migrants in Geneva. This descriptive cross sectional study included consecutive undocumented migrants presenting from November 2007 to February 2008 to a health facility offering free access to health care to this population. Following informed consent, they completed a self administered questionnaire about their socio-demographic profile and sexual and reproductive health behaviors. A total of 384 patients were eligible for the study. 313 (82%) agreed to participate of which 77% (241 patients) completed the survey. Participants were mainly young, Latino-American, single, well-educated and currently working women. They had multiple partners and reported frequently engaging in sexual intercourse. Use of contraceptive methods and strategies of prevention against sexually transmitted infections (STI) were rare. Nearly half of the women had had at least one induced abortion and 40% had had an unplanned pregnancy. One in four participants reported a current or past STI or other genital infection. The results of our study suggest that undocumented migrants engage in frequent and high risk sexual intercourse with insufficient use of contraceptive methods and suboptimal strategies of prevention against STI. Our study underlines the real need for specific sexual and reproductive educational programs targeting this hard to reach population.