In 2015, the US Food and Drug Administration published revised guidance that recommended a change in blood donor deferral of men who have sex with men (MSM) from an indefinite to a 12-month deferral since the donor last had sex with a man. We assessed whether HIV incidence in first-time blood donors or associated transfusion risk increased. Donations in 4 major blood collection organizations were monitored for 15 months before and 2 years after implementation of the 12-month MSM deferral policy. HIV-positive donations were classified as recently acquired or long-term using a recent infection testing algorithm and incidence in both periods estimated. Residual transfusion transmission risk was estimated by multiplying incidence by the length of the infectious window period. The latter was estimated using a model based on infectious dose and the sensitivity of nucleic acid testing. Factors associated with incident infection in each period were assessed using Poisson regression. Overall HIV incidence in first-time donors before implementation of the 12-month MSM deferral was estimated at 2.62 cases per 100 000 person-years (105 PY) (95% credible interval [CI], 1.53-3.93 cases/105 PY), and after implementation at 2.85 cases/105 PY (95% CI, 1.96-3.93 cases/105 PY), with no statistically significant change. In male first-time donors, the incidence difference was 0.93 cases/105 PY (95% CI, -1.74-3.58 cases/105 PY). The residual risk of HIV transfusion transmission through components sourced from first-time donors was estimated at 0.32 transmissions per million (106) packed red blood cell transfusions (95% CI, 0.29-0.65 transmissions/106 transfusions) before and 0.35 transmissions/106 transfusions (95% CI, 0.31-0.65 transmissions/106 transfusions) after implementation. The difference was not statistically significant. Factors associated with incident infection were the same in each period. We observed no increase in HIV incidence or HIV transfusion transmission risk after implementation of a 12-month MSM deferral policy.