Background: Randomised controlled trials often rely on questionnaires for follow-up.

Objective: To compare response rates to an online and postal 12-month follow-up questionnaire on sexual health in female students who took part in a chlamydia screening trial.

Methods: 1329 sexually active female students aged 16–27 were recruited from 12 universities and further education (FE) colleges. The 299 participants recruited between September 2004 and February 2005 were sent a postal questionnaire after 12 months. The 1030 participants recruited between March and December 2005 were contacted by email after 12 months and given a weblink to an online questionnaire.

Results: The response rates to the 12-month questionnaire in the online and postal groups were 51% and 29% 4 weeks after follow-up commenced (RR 1.78 (1.47 to 2.14)) and 72% and 59% after 3 months. After adjusting for ethnicity, smoking, type of educational institution (university or FE college) and subject studied (health-related or not), the RR at 4 weeks was 1.88 (1.42 to 2.50). However, a prior telephone call to confirm contact details increased the response rate at 3 months in the postal group. In the online group, university students, those of white ethnicity and non-smokers had higher response rates at 4 weeks.

Conclusions: In this young student population, an online questionnaire was quicker, cheaper and more efficient than a postal questionnaire. However, some FE college students did not have an email address. Telephone prompts and postal questionnaires were essential in obtaining a good response rate.