Introduction No consent for health and medical research is appropriate when the criteria for a waiver of consent are met, yet some ethics committees and data custodians still require informed consent. Methods A single-blind parallel-group randomised controlled trial: 1129 families of children born at a South Australian hospital were sent information explaining data linkage of childhood immunisation and hospital records for vaccine safety surveillance with 4 weeks to opt in or opt out by reply form, telephone or email. A subsequent (...) telephone interview gauged the intent of 1026 parents (91%) in relation to their actions and the sociodemographic differences between participants and non-participants in each arm. Results The participation rate was 21% (n=120/564) in the opt-in arm and 96% (n=540/565) in the opt-out arm (χ2 (1 df) = 567.7, p<0.001). Participants in the opt-in arm were more likely than non-participants to be older, married/de facto, university educated and of higher socioeconomic status. Participants in the opt-out arm were similar to non-participants, except men were more likely to opt out. Substantial proportions did not receive, understand or properly consider study invitations, and opting in or opting out behaviour was often at odds with parents' stated underlying intentions. Conclusions The opt-in approach resulted in low participation and a biased sample that would render any subsequent data linkage unfeasible, while the opt-out approach achieved high participation and a representative sample. The waiver of consent afforded under current privacy regulations for data linkage studies meeting all appropriate criteria should be granted by ethics committees, and supported by data custodians. Trial registration number Australian New Zealand Clinical Trials Registry ACTRN12610000332022. (shrink)