OBJECTIVE: To explore compliance with the UK blood services' criterion that excludes men who have had penetrative sex with a man from donating blood, and to assess the possible effects of revising this policy. DESIGN: A random location, cross sectional survey followed by qualitative interviews. SETTING: Britain. PARTICIPANTS: 1028 of 32,373 men in the general population reporting any male sexual contact completed the survey. Additional questions were asked of a general population sample (n=3914). Thirty men who had had penetrative sex with a man participated in the qualitative interviews (19 who had complied with the blood services' exclusion criterion and 11 who had not complied). Main outcome measure Compliance with the blood services' lifetime exclusion criterion for men who have had penetrative sex with a man. RESULTS: 10.6% of men with experience of penetrative sex with a man reported having donated blood in Britain while ineligible under the exclusion criterion, and 2.5% had donated in the previous 12 months. Ineligible donation was less common among men who had had penetrative sex with a man recently (in previous 12 months) than among men for whom this last occurred longer ago. Reasons for non-compliance with the exclusion included self categorisation as low risk, discounting the sexual experience that barred donation, belief in the infallibility of blood screening, concerns about confidentiality, and misunderstanding or perceived inequity of the rule. Although blood donation was rarely viewed as a "right," potential donors were seen as entitled to a considered assessment of risk. A one year deferral since last male penetrative sex was considered by study participants to be generally feasible, equitable, and acceptable. CONCLUSIONS: A minority of men who have sex with men who are ineligible to donate blood under the current donor exclusion in Britain have nevertheless done so in the past 12 months. Many of the reasons identified for non-compliance seem amenable to intervention. A clearly rationalised and communicated one year donor deferral is likely to be welcomed by most men who have sex with men.