Background A register- and Internet-based chlamydia screening started in 2008 among 16–29&emsp14;year olds in Amsterdam, Rotterdam and South-Limburg, aiming to reduce population prevalence of chlamydia infections. Automated respondent reminders by letter, email and SMS were used to encourage and optimise participation. The contribution of reminders on package request and sample return was examined in relation to characteristics of the target population. Methods On average, 280 000 persons were annually invited by means of a personal letter. Individuals who did not respond to the invitation letter received a reminder letter after 1&emsp14;month. Email- and SMS reminders were sent to individuals who requested a test kit and did not return a sample or did not check their test results online. We evaluated the effect of reminders on response and participation rates in two screening rounds. Logistic regression analyses were conducted to identify determinants of providing a GSM number, late response (requesting a test package after a reminder letter), and late participation (returning a sample after email/SMS reminders). Results The overall package request rate in the first round increased from 12% to 20% (see <cross-ref type="fig" refid="fig1">Abstract P2-S8.06 Figure 1</cross-ref>) after the reminder letter (in round 2 from 8% to 14%). The proportion of individuals returning a sample increased from 10% to 16% after email/SMS reminders (in round 2 from 7% to 11%). Of all respondents, 99% provided an email address and 72% a GSM number. Factors associated with providing a GSM number were younger age (16–24&emsp14;yrs), non-Dutch background, lower educational level, symptoms of an STI, and sexual risk behaviour in the past 6&emsp14;months (p<0.05). Determinants for late response (requesting a test package after reminder letter) were male gender, young age (16–19&emsp14;yrs), non-Dutch background, having a casual partner or ≥2 sex partners in ≤6&emsp14;months. The email/SMS reminders ...