Background and Objectives: Persons who participate in behaviors such as drug use and buying or selling sex are at elevated risk for sexually transmitted diseases (STD)/human immunodeficiency virus (HIV) infection. Goals: To describe the economic structure supporting drug use and the buying and selling of sex services in two urban Houston communities. Study Design: Residents of two Houston communities participated in street-intercept interviews to obtain information on sex and drug use behaviors. Results: Many members of the population reported having bought or sold sex. A history of crack use significantly predicted the trading of sex for money and drugs, and sellers of sex were more likely to have engaged in recent high-risk sexual behavior than those who had never sold sex. Conclusion: The data are suggestive of an underground economy for the exchange of sex for drugs or money, the existence of which facilitates the spread of STDs in high-risk communities.