In May-June 2002, 4,974 men who have sex with men (MSM), average age 31 years, completed a self-administered questionnaire online accessed through two popular gay Internet sites in the UK (gaydar and gay.com UK). Most men were white (95%), employed (83%), lived in the UK (91%) and self-identified as gay (85%). Nearly half (46%) had not been tested for HIV, while 6% said they were HIV-positive. One-third (31%) reported high-risk sexual behaviour in the previous three months, i.e. unprotected anal intercourse with a partner of unknown or discordant HIV status. Nearly all the men (82%) had looked for a sexual partner on the Internet and three-quarters had been doing so for more than a year. Almost half the men (47%) said they preferred to meet men through the Internet rather than in bars or other 'offline' venues. Although nearly 40% of men said the most important reason for using these Internet sites was to find sexual partners, a further 17% said they primarily used them to have contact with other men, 16% because they were bored, 12% for entertainment, 4% because they were lonely and 3% because they were addicted to it. The majority of men in this survey had favourable attitudes towards online health promotion. Most men thought that Internet sites should allow health workers into chat-rooms (75%); would click on a banner to find out about sexual health (78%); and said if they met a health worker in a chat-room they would find out what they had to say (84%). In multivariate analysis, being HIV-positive, older age and high-risk sexual behaviour were all independently associated with an increased frequency of using the Internet to look for sex (p<0.05). The Internet has emerged as an important meeting place for men who have sex with men. As online HIV prevention initiatives are developed it will be important to monitor the extent to which the favourable attitudes seen here are reflected in preventive behaviours.