The incidence of sexually transmitted diseases (STDs) in the United States is among the highest in the western industrialized world. Nearly 19 million new STDs are diagnosed each year, and more than 65 million Americans live with an incurable STD, such as herpes and human papillomavirus (HPV). Young people, in particular, are at a heightened risk of acquiring an STD The Centers for Disease Control and Prevention (CDC) estimates that those between the ages of 15 and 24 account for about one-half of the new STDs diagnosed every year, although this age group represents only one-quarter of the sexually active population. The health costs of STDs are high. For example, untreated chlamydia and gonorrhea--the two most common reportable STDs--can cause significant health problems, including infertility, pregnancy complications, and increased risk of HIV infection. And some strains of HPV, which is widespread, are linked to cervical cancer. The economic costs are also high. The lifetime medical costs of STD incidence to young adults who were between the ages of 15 and 26 in 2000 were estimated to be $6.5 billion. This "Research Brief" presents updated information on STDs among young adults, a major public health challenge that calls for creative solutions. To produce the brief, Child Trends analyzed recently released data from the National Longitudinal Study of Adolescent Health (Add Health) to provide estimates on the prevalence of and attitudes towards STDs among young adults, as well as on the behaviors that may put youth at risk of contracting an STD. We found that 15 percent of young adults between the ages of 18 and 26 have had an STD within the past year, with that proportion varying by gender, race/ethnicity, and relationship status. We also found that despite the high prevalence of STDs within this age group, most young adults did not perceive that they were at risk. Perhaps even more troubling, we found that large numbers of young adults continue to engage in behaviors that do put them at risk of contracting an STD. (Contains 9 figures and 4 footnotes.)