Formal and informal commercial sex work is a way of life for many poor women in developing countries. Though sex workers have long been identiÖed as crucial in a§ecting the spread of HIV/AIDS, particularly in sub-Saharan Africa, the nature of sex-for-money transactions remains poorly understood. This paper investigates sex worker behavior using daily self-reported data on sexual behavior, income shocks, expenditures, and labor supply for a sample of 237 women in Western Kenya. We Önd signiÖcant day-to-day áuctuations in sex worker decisions, and that women engage in sex-for-money transactions in part to deal with unexpected non-labor income shocks. Riskier sex is better compensated in Western Kenya, and we Önd that women increase their supply of riskier, better compensated sex on days in which a household member falls ill. In particular, women are 23.6% more likely to have unprotected sex, 16.8% more likely to have anal sex, and increase the number of unprotected sexual acts by 21.7% on such days. These increases in risky sexual behavior have important health consequences for these women and on the spread of HIV/AIDS. While not denying the need for interventions that encourage women to leave the commercial sex industry, our research suggests that important opportunities exist to reduce the health risks of sex work within sex work beyond HIV education and condom distribution