The prevalence of HIV/sexually transmitted diseases (STDs) was determined, the risk characteristics examined, and factors associated with HIV infection identified among noninstitutionalized female sex workers (FSWs), using a cross-sectional survey with 2-stage cluster sampling. Four hundred FSWs were interviewed face to face using a structured questionnaire and tested for HIV, syphilis, Chlamydia infection, and gonorrhea. HIV seroprevalence was 12%, syphilis 17% (using the treponemal pallidum hemagglutination assay), Chlamydia infection 3.8% (using polymerase chain reaction [PCR]), and gonorrhea 6.3% (PCR). Lower-class FSWs averaged 2 clients per day, and middle-class FSWs about 1.2. Median duration in sex work was 2.3 years. Consistent condom use was 63% with irregular clients, 41% with regular clients, and only 4.8% with "love mates." Fifty-five percent had had sex with a drug user(s). Thirty-eight percent used drugs, of whom 83% injected. Factors associated with HIV included being young, having a low level of education, longer residence in Hanoi, being a lower-class FSW, having higher income compared with peers, perception of self being at low risk for HIV, poor knowledge of HIV, and sharing injecting equipment. Intervention strategies should include reduction of both stigmatization and sharing of drug paraphernalia, promotion of nonstigmatizing voluntary testing and counseling, and aggressive marketing and promotion of condoms.