Because of concerns for HIV risks and need to plan effective programs, we assessed the number and risks of sex workers in Nha Trang City, Vietnam. Sex workers were contacted in streets, beaches, bars, and restaurants, and a capture-recapture method was used to estimate their number. An estimated 444 women worked on the streets and beach ("direct" sex workers) and 486 worked in bars and restaurants or other facilities ("indirect" sex workers). Direct and indirect sex workers engaged in sex work primarily to support their families. Direct sex workers were older and were more at risk for HIV risk than were indirect sex workers. Direct sex workers had more clients, were less likely to report always using condoms (67% vs. 81%), more likely to report a prior sexually transmitted infection (19% vs. 16%), and more likely to have clients who inject drugs (16% vs. 13%). This assessment has implications for planning programs to reduce sex work and its risks in Vietnam and potentially other countries.