Herpes simplex virus type 2 (HSV-2) is one of the most common sexually transmitted infections worldwide and the primary cause of genital and neonatal herpes and genital ulcer disease. Multiple studies have shown that HSV-2 infection increases the risk for human immunodeficiency virus (HIV) infection by at least twofold. HSV-2 infection is lifelong, and serologic testing provides the best method to estimate HSV-2 prevalence. Since 1976, CDC has monitored HSV-2 seroprevalence in the United States through the National Health and Nutrition Examination Survey (NHANES). After increasing from 1976-1980 (NHANES II) to 1988--1994 (NHANES III), HSV-2 seroprevalence decreased, from 21.0% in 1988-1994 to 17.0% in NHANES 1999-2004. To determine whether HSV-2 seroprevalence in the United States has changed since 1999-2004 and to estimate HSV-2 seroprevalence by age, race/ethnicity, and reported lifetime number of sex partners, CDC analyzed serologic test results from persons aged 14-49 years who participated in NHANES 2005-2008. The results indicated that HSV-2 seroprevalence was 16.2% overall, not statistically different from the seroprevalence in 1999-2004. Seroprevalence was highest among women (20.9%) and non-Hispanic blacks (39.2%). Of those infected with HSV-2, 81.1% had not received a diagnosis. Clinicians, health departments, health-care organizations, and community groups should promote measures that prevent HSV-2 transmission, including minimizing the number of sex partners, avoiding concurrent sexual partnerships, and using condoms consistently and correctly. Patients with known HSV-2 infection should be tested for HIV.