OBJECTIVES: To explore attitudes toward circumcision among men who have sex with men (MSM) in London and the feasibility of conducting research into circumcision and HIV prevention in this population. METHODS: A convenience sample of MSM visiting central London gyms completed a confidential, self-administered questionnaire between May and June 2008. Information was collected on participants' demographic characteristics, self-reported HIV status, sexual behavior, circumcision status, attitudes toward circumcision, and willingness to participate in research on circumcision and HIV prevention. RESULTS: Of 653 MSM, 29.0% reported that they were circumcised. Overall, HIV prevalence was 23.3%; this did not differ significantly between circumcised and uncircumcised men (18.6% vs. 25.2%, respectively; adjusted odds ratio 0.79, 95% confidence interval: 0.50-1.26). A similar proportion of circumcised and uncircumcised men reported unprotected anal intercourse in the previous 3 months (38.8% vs. 36.7%, adjusted odds ratio 1.06, 95% confidence interval: 0.72-1.55). Uncircumcised men were less likely to think that there were benefits of circumcision than circumcised men (31.2% vs. 65.4, P < 0.001). Only 10.3% of uncircumcised men said that they would be willing to participate in research on circumcision as an HIV prevention strategy. CONCLUSIONS: Most uncircumcised MSM in this London survey were unwilling to participate in research on circumcision and HIV prevention. Only a minority of uncircumcised men thought that there were benefits of circumcision. It is unlikely that circumcision would be a feasible strategy for HIV prevention among MSM in London.