BACKGROUND Sexual acquisition of HIV is influenced by choice of partner, sex act, and condom use. However, current risk-reduction strategies focus mainly on condom use. GOAL To estimate the contribution of choice of partner, sex act, and condom use on the per-act relative and absolute risks for HIV infection. STUDY DESIGN Per-act relative risk for HIV infection was calculated with use of estimates of HIV prevalence, risk of condom failure, HIV test accuracy, and per-act risk of HIV transmission for different sex acts. Absolute risks were calculated on the basis of these relative risk estimates. RESULTS Choosing a partner who tested negative instead of an untested partner reduced the relative risk of HIV infection 47-fold; using condoms, 20-fold; and choosing insertive fellatio rather than insertive anal sex, 13-fold. Choosing one risk-reduction behavior substantially reduces absolute risk of HIV infection for heterosexuals but not for men who have sex with men. CONCLUSION Clarifying the magnitude of risk associated with different choices may help people make effective and sustainable changes in behavior.