Heterosexual women (n = 330), heterosexual men (n = 319) and gay men (n = 331) attending a London HIV-testing clinic in 2002-03 completed a confidential self-administered questionnaire concerning their sexual behaviour and use of the Internet for seeking sexual partners (response rate 70%). One-in-twenty (5%) heterosexual women and one-in-ten (10%) heterosexual men had used the Internet to look for sexual partners in the previous 12 months compared with nearly half (43%) the gay men (P<0.001). Rates of high-risk sexual behaviour with a casual partner were elevated among those who used the Internet to look for sex (compared with those who did not). However, people who looked for sex through the Internet were just as likely to meet their high-risk casual partners offline as online. Our findings suggest that high-risk heterosexual women and men are selectively using the Internet to look for sex rather than the Internet per se leading to riskier sexual behaviour.