This is a phase III, multicenter trial to assess the safety and effectiveness of 1% tenofovir gel, administered vaginally by approximately 2900 sexually active women at high risk for sexually transmitted HIV. Approximately 2600 women aged 18-30 years old will be enrolled to achieve the required number of endpoints to show an effect on HIV-1 infection, while up to 300 additional women aged 31-40 years old will be enrolled to collect more safety information in this age group.