Formal and informal commercial sex work is a way of life for many poor women in developing countries. Though sex workers have long been identified as crucial in affecting the spread of HIV/AIDS, particularly in Sub-Saharan Africa, the nature of sex-for-money transactions remains poorly understood. Using a unique panel dataset constructed from 192 self-reported sex worker diaries which include detailed information on sexual behavior, labor supply, and health shocks, the authors find that sex workers adjust their supply of risky, better compensated sex to cope with unexpected health shocks, exposing themselves to increased risk of HIV infection. In particular, women are 3.1 percent more likely to see a client, 21.2 percent more likely to have anal sex, and 19.1 percent more likely to have unprotected sex on days in which a household member falls ill. Women also increase their supply of risky sex on days after missing work due to symptoms from a sexually transmitted infection. Given that HIV prevalence has been estimated at 9.8 percent in this part of Kenya, these behavioral responses entail significant health risks for sex workers and their partners, and suggest that sex workers are unable to cope with risk through other formal or informal consumption smoothing mechanisms.