Despite a recent reduction in the number of human immunodeficiency virus (HIV) infections attributed to injecting drug use in the United States, 9% of new U.S. HIV infections in 2009 occurred among injecting drug users (IDUs). To monitor HIV-associated behaviors and HIV prevalence among IDUs, CDC's National HIV Behavioral Surveillance System (NHBS) conducts interviews and HIV testing in selected metropolitan statistical areas (MSAs). This report summarizes data from 10,073 IDUs interviewed and tested in 20 MSAs in 2009. Of IDUs tested, 9% had a positive HIV test result, and 45% of those testing positive were unaware of their infection. Among the 9,565 IDUs with HIV negative or unknown HIV status before the survey, 69% reported having unprotected vaginal sex, 34% reported sharing syringes, and 23% reported having unprotected heterosexual anal sex during the 12 previous months. Although these risk behavior prevalences appear to warrant increased access to HIV testing and prevention services, for the previous 12-month period, only 49% of the IDUs at risk for acquiring HIV infection reported having been tested for HIV, and 19% reported participating in a behavioral intervention. Increased HIV prevention and testing efforts are needed to further reduce HIV infections among IDUs.