To study HIV-1 seroprevalence, risk factors, and preventive behaviors among reproductive-age women in northern Thailand, 804 consenting women who were identified postpartum or who were visiting family planning clinics were interviewed and tested during 1998 to 1999. Almost all women were currently married and had been pregnant more than once. Their median age was 27 years. HIV-1 seroprevalence was 3.1% overall and was higher in women aged between 25 and 29 years (5.9%), having had ≥2 lifetime sex partners (6.5%), or whose current marriage had lasted for ≤1 year (7.0%). No woman reported HIV risk factors other than heterosexual sex. Most (76%) HIV-infected women reported no casual sex partners and, therefore, had likely acquired the infection from their husbands. HIV testing and partner communications were common, but only 2% of couples used condoms consistently in the prior 6 months. Nearly half of these women perceived themselves at no or low risk for HIV infection; these women were less likely to have taken preventive actions. To prevent HIV transmission in stable partnerships in this population, additional efforts are needed to increase HIV testing and condom use, to improve women's negotiation skills, and to develop new methods that do not require partner cooperation such as vaginal microbicides or vaccines.