This study aimed to characterize the profiles of injecting drug users (IDUs) who were unaware of their HIV serostatus, given the importance of this information for prevention strategies, especially in this vulnerable population, key to the HIV/AIDS dynamic. As part of a cross-sectional multi-city survey, IDUs were interviewed and HIV-tested by the ELISA technique. IDUs were categorized according to knowledge of their own HIV status as either aware or unaware. Means, averages, and proportions were compared between the groups using biand multivariate analyses. Of 857 IDUs interviewed, 34.2% were unaware of their HIV serostatus. Those who were unaware were more likely: to have been recruited at sites where the HIV prevalence rate was considered medium (> 10 to 50%; odds ratio = 8.0) or high (> 50%; 4.0); to be illiterate (OR = 4.54); to have no prior HIV test (OR = 2.22); to be male (OR = 1.81); and to have been enrolled more recently in syringe-exchange programs (OR = 1.69). HIV prevention programs should target both individuals at risk and HIV-positive individuals. Programs to expand access to HIV testing are pivotal and should be tailored to specific contexts and populations. AIDS Serodiagnosis; Intravenous Substance Abuse; Acquired Immunodeficiency Syndrome; HIV Introduction Public health strategies have been developed to increase awareness of HIV serostatus among individuals and populations for the control of the HIV/AIDS epidemic. Knowledge of one’s serostatus, expressed as early diagnosis of HIV infection, can allow adequate access to treatment, ensuring better response to therapy and quality of life among persons with HIV as well as reduction in AIDS mortality 1. In addition, it is believed that such awareness is associated with lower frequency of practices that expose individuals to the risk of being (re)infected and of transmitting the virus 2,3,4,5,6. Testing, counseling, and returning the test results thus has an effect on the epidemic’s dynamics by reducing the frequency of risk interactions and HIV transmission 2,3,4. Statistics indicate 40 million individuals infected worldwide 7, with 600 thousand infected in Brazil 8. Of these, an estimated 67% are unaware of their HIV serostatus 8. Based on this scenario and studies emphasizing the advantages of serostatus awareness, since the late 1980s the Brazilian Ministry of Health has invested in the creation of Anonymous Testing Centers (ATCs) or Centers for Serological Support and Orientation, characterized by free access, voluntary participation, and confidentiality, offering HIV testing along with preand post-test counseling 2. In addiFerreira AD et al. 816 Cad. Saúde Pública, Rio de Janeiro, 22(4):815-826, abr, 2006 tion to the availability of these centers, campaigns like Fique Sabendo! (Know it!) have also been developed. This campaign was launched in 2003 and aims to raise the Brazilian population’s awareness concerning the importance of HIV testing, with the goal of a 150% increase in the number of tests performed in the country by 2006 8. The epidemiological profile of HIV/AIDS infection in Brazil is known to be differentiated, varying according to the period analyzed, exposure categories, and regional characteristics. Among the exposure categories, the most relevant is “heterosexual”, followed by “injecting drug user” (IDUs), accounting for 19.1% of the AIDS cases diagnosed as of 2004 9. However, the proportional participation of IDUs varies substantially between regions, corresponding to some 60% of the AIDS cases reported in the South of the country 7,10,11. IDUs play an even more relevant role in the HIV/ AIDS dynamics when considering the risk of transmission from infected IDUs to their sex partners and offspring 11. Thus, injecting drug use behaviors such as initiation of use, sharing of injecting materials, and type of drug used have contributed to the spread of HIV and maintenance of the epidemic in Brazil 12. The present study aims to characterize the profile of Brazilian IDUs who are unaware (versus aware) of their HIV serostatus, using data from the AjUDE-Brasil II Project. This line of analysis was allowed by the range of information included in the project and is especially relevant because of the lack of quantitative analyses on the choice to test for HIV infection in specific populations in the country. The resulting information can support public policies for testing and counseling, fostering awarenessraising strategies and contributing to greater effectiveness in the interventions to control the HIV/AIDS epidemic in Brazil.