The purpose of this study is to examine the efficacy of a theory based, online tailored intervention on stimulating safe-sex practices among men who have sex with men (MSM). Our hypothesis was that a tailored internet intervention would be more effective in stimulating safe-sex practices of gay men than a non-tailored internet intervention when compared to a waiting-list control group.

Relationships are a high-risk setting for HIV-infection. This trial aimed at testing the efficacy of an online theory-based tailored intervention for preparing single MSM to practice safe sex with future steady partners-labeling it the 'cognitive vaccine approach'.

The target was the promotion of negotiated safety (NS): steady partners testing for HIV and reaching agreements to either be monogamous or to only have safe sex outside the relationship in order to have safe unprotected anal intercourse with each other. The intervention content was based on the information, motivation, behavioral-skills model and the intervention was tailored according to knowledge, motivation, and skill-related deficiencies of each participant. Condom use was promoted as the default alternative for NS. Using an online randomized controlled trial we examined the effects of a tailored versus non-tailored version of the intervention. The cognitive effect (i.e. response efficacy, intentions, and perceived behavioral control (PBC) was measured directly after the intervention and, after 6-months, the behavioral effect (i.e. NS and condom use) via e-mail follow-up.

Inclusion Criteria:

• Men who have sex with men of all ages, HIV-negative or of unknown serostatus, single, and open to a steady relationship with a man in the future