Objective. Cohort studies must collect data from their participants as economically as possible, while maintaining response rates. This randomised controlled trial investigated whether offering a choice of online or paper questionnaires resulted in improved response rates compared with offering online first. Study Design and Setting. Eligible participants were young people in the ALSPAC study (born 01/04/91-31/12/92 in the Avon area). After exclusions, 8,795 participants were randomised. The ‘online first’ group were invited to complete the questionnaire online. The ‘choice’ group were also sent a paper questionnaire, and offered a choice of completion method. The trial was embedded within routine data collection. The main outcome measure was the number of questionnaires returned. Data on costs were also collected. Results. Those in the 'online first' arm of the trial were less likely to return a questionnaire (adjusted odds ratio 0.90, 95% CI 0.82-0.99). The ‘choice’ arm was more expensive (mean difference per participant £0.71, 95% CI £0.65-£0.76). It cost an extra £47 to have one extra person complete the questionnaire in the ‘online first’ arm. Conclusions. Offering a choice of completion methods (paper or online) for questionnaires in ALSPAC increased response rates but was more expensive than offering online first.