Background: In the Netherlands, a national chlamydia screening program started in 2008, but the participation was low and the screening was not cost-effective. This study aimed to explore unconscious and conscious associations with chlamydia screening (16-29 year-olds). In addition, we examined whether information presented in chlamydia screening invitation letters had an effect on the evaluation of these determinants compared to a no-letter group. Methods: An Internet survey was conducted that included self-report measures of attitude, susceptibility, severity, unrealistic optimism, subjective, moral, and descriptive norm, perceived behavioral control, outcome expectations, barriers, intention, and a response time measure to assess unconscious associations of chlamydia screening with annoyance, threat and reassurance. Results: On the unconscious level, participants (N = 713) who received no information letter associated testing for chlamydia with annoyance and threat, but also with reassurance (all p's <.001). On the self-report measures, participants showed a low intention towards chlamydia screening (M = 1.42, range 1-5). Subjective norm, moral norm, perceived susceptibility and attitude were the most important predictors of the intention to screen (R-2 = .56). Participants who rated their susceptibility as high also reported more risky behaviors (p <.001). In the groups that received a letter (N = 735), a weaker unconscious association of chlamydia screening with annoyance was found compared with the no-letter group (p <.001), but no differences were found in reassurance or threat. Furthermore, the letters caused a higher intention (p <.001), but intention remained low (M = 1.74). On a conscious level, giving information caused a more positive attitude, higher susceptibility, a higher subjective and moral norm, and more positive outcome expectations (all p's <.001). Conclusion: Subjective norm, moral norm, susceptibility, and attitude towards chlamydia might be crucial targets to increase ...