This paper examines the impacts of four abstinence-only education programs on adolescent sexual activity and risks of pregnancy and sexually transmitted diseases (STDs). Based on an experimental design, the impact analysis uses survey data collected in 2005 and early 2006 from more than 2,000 teens who had been randomly assigned to either a program group that was eligible to participate in one of the four programs or a control group that was not. The findings show no significant impact on teen sexual activity, no differences in rates of unprotected sex, and some impacts on knowledge of STDs and perceived effectiveness of condoms and birth control pills. (Contains 1 figure, 9 tables and 1 footnote.)