With the help of a community-based survey, we assess the potential effect of circumcision on the HIV epidemic among men who have sex with men (MSM) in San Francisco. Only a small minority of MSM would both derive benefit from circumcision (i.e., were uncircumcised, HIV-negative, predominantly insertive, and reported unprotected insertive anal sex) and be willing to participate in circumcision trials (0.7%) or be circumcised if proven effective as a prevention strategy (0.9%). Circumcision would have limited public health significance for MSM in San Francisco.