Herpes infection is an important co-factor in HIV transmission. A recent meta-analysis demonstrated that HSV infection contributed to a 2-fold increased risk of HIV transmission. Suppressive therapy with acyclovir has been a method proposed to reduce HIV transmission. We are conducting a randomized controlled cross-over trial to evaluate the effect of acyclovir suppressive therapy on HIV genital shedding in women co-infected with HIV-1 and Herpes Simplex Virus Type-2 (HSV-2) in Thailand.