In this paper, we exploit a lottery in Tanzania, which randomly assigned eligible participants to receive $100 cash grants. The randomized nature of the lottery allows us to estimate the causal impact of positive income shocks on risky sexual behavior. We found that winning the lottery led men to have 0.28 (95 % CI 0.14, 0.55) more sexual partners and to a 0.21 (95 % CI 0.01-0.4) increase in the probability of unprotected sex with a non-primary partner relative to a control group of eligible non-winners. We found no significant effect of winning the lottery on the sexual behavior of women.