The aim of this study was to describe the sexual behaviour of men who had sex with both men and women in the preceding five years (behaviourally bisexual men), specifically to examine their needs in relation to HIV prevention. Anonymous telephone interviews were carried out with respondents recruited though advertisements in UK newspapers and magazines with (sexual) 'personal' or 'contact' sections. Data are reported from 745 respondents. Respondents report relatively high and approximately equal numbers of male and female sexual partners in the year preceding interview. There is a clear patterning of sexual activity by type of partner (regular or casual). A high proportion report anal intercourse with female and male partners. A third disclose their homosexual practices to regular female partners. Although self-reported HIV seroprevalence is low (less than 1%), the levels of unprotected sex with multiple sexual partners indicate substantial potential for transmission of sexually transmitted infections, including HIV. While low seroprevalence suggests that there is little overlap with existing core groups of HIV infection, the study provides information to judge the priority of targeting HIV prevention initiatives and suggests ways in which initiatives could be undertaken. PIP: Findings are reported from an investigation of the sex and HIV risk reduction behaviors during the past 5 years of behaviorally bisexual men in the UK, men who have sex with both men and women regardless of their stated sexual identity. Anonymous telephone interviews were conducted with 745 respondents recruited through advertisements in UK newspapers and magazines with sexual "personal" or "contact" sections. The final sample of respondents from 107 postcode regions of the country was obtained from 60,368 ineffective calls and 7398 effective calls made to the survey hotline over an 8-week period. The men were 16-73 years old, of median and mean ages 32 and 33.3 years, respectively. The sample closely approximated the UK general population of men in terms of educational status and social and occupational classes, but slightly under-represented Blacks, other ethnic minorities, and married men. 1 man reported being diagnosed with HIV infection and 9.1% had attended a STD clinic in the preceding year. In the year before the interview, although 94.7% had female partners and 91.9% had male partners, only 43.9% described themselves as bisexual and less than 1% identified as gay or homosexual. The men more often reported having regular female partners than regular male partners, and casual female partners were reported by more men than casual male partners. 67.1% of the 93.8% of men who reported having vaginal intercourse used no condom. 22.6% of the 44.4% of men who reported having anal intercourse with women used no condom. 18.2% of the 62.2% men who had anal sex with men used no condom. 33% disclose their homosexual behavior to regular female partners.